Yay, Pudding, Evidence (and some Thesis SPOILERS)

[Meisatsu]

Here is some textual/logical evidence of the UD to DP aspect:

• There's RiffRaff, the wolf, then Wulf, also a wolf, and originally an enemy
• Technus in all versions is a dead ringer for Simon Bar Sinister
• Dash Baxter is the reincarnation of Cad Lackey, who was himself bullied immensely, hence, karmic retribution
• Look at Underdog's eyes and eyebrows -- the exact same shape, the roundness, the blockiness!
• In an episode called "The Big Shrink", Underdog and Sweet Polly end up being shrunk to thumb-size and come face to face with "giant" rats (recreated somewhat here) -- which is echoed briefly, albeit with Danny and Dash, in the Danny Phantom episode "Micro-Management"; also, Underdog's powers (like Danny's) are weakened by the miniature state

...and here is some more/further confiriming visual evidence:



If Danny's logo, and here, brief pose, isn't enough, then consider his appearance in the episode Identity Crisis.



Not only did he don a cape, but he appeared before an American flag. Underdog never appeared before one, HOWEVER, a pair of them always rose in his eyes whenever he consumed the rejuvenating Super-Energy Pill.

Look, Ma, no legs! A pic of the two looking similar in scenes from their respective opening sequences...



Both using the 'Pile Driver Fist' technique against giant robots...



This proves beyond a doubt that Underdog is Danny Phantom, Danny's ghost-half.

...

As for his human self, and the IY-DP aspect...



The one major contrast between Human Inuyasha (as he was then) and Danny Fenton is that the latter can actually make use of some of his powers in human form. Other than that, in addition to their hair color, their personalities are strikingly similar; both are more emotional, and prone to admitting how they feel, though never thinking to back down from a fight should it present itself.

Don't think about it TOO hard, now...



Danny and Inuyasha seem closer than Danny and Underdog, for the fact that the former both have two distinct forms -- but then, if Danny Phantom is in fact Underdog, then how did his presence manifest itself in Inuyasha? ... was it partial (alternating), or full?

Talk about your chronic back pain...



...fwapped against a rocky cliff, by the same enemy, no less!

[Meisatsu]

Danny isn't the only one with a deep spiritual heritage: to reiterate, the prideful jock, Dash has the aura of the rather acquiescent Cad Lackey --



The non-physical tie is this: Cad was bullied relentlessly, not only by Underdog, but even by Simon. Hence, at long last, Cad has become the bully.



Similarly, before Dash, Cad was incarnate in Myouga the flea, who was frequently flattened by Inuyasha himself. Dash's cry of "I don't do puny!" in Micro-Management was likely a subconscious utterance of lament for his most recent past. It may also be why he takes a special pleasure in beating up Danny.

Speaking of Simon Barsinister, again, he is manifest vaguely in the looks but strongly in the voice and personality of Nicolai Technus, especially from his first appearance --



He was later incarnate in the elder Thunder Brother, Hiten...



Then, again, there is the ghost called Wulf, whose transmigration needs no explanation...

[Meisatsu]

Tucker Foley may not have a habit of blatantly grabbing women's butts, but his amorous air is entirely familiar...



In King Tuck, Tucker was reacquainted with the reincarnated shakujou, albeit that its holy powers had been tainted...



One would think that Sesshomaru would remain Inuyasha's sibling, but instead, Danny's sister Jasmine a.k.a. Jazz is the reincarnation of Shippou (!) --



Of course, the ambitious ghost-hunter Valerie is the reincarnation of the youkai taijiya "youkai exterminator" Sango.



Both have two outfits apiece: one casual, and one for fighting.




The size-shifting ghost dog, known as "Cujo" (later "Cujojo") after Danny referred to him by the name, is the reincarnation of Sango's pet size-shifting kitsune, Kirara.

[Meisatsu]

Because of Kikyou's initial reincarnation, her spiritual lineage becomes a bit confusing, but for certain, she is the reincarnation of Sweet Polly...




Polly's aura is present in Kikyou until the time of her first death. Then, it seems that Polly's purity is passed on to Kagome -- and eventually to Ruth.




The gloom, the many mixed souls, and the rest of Kikyou's essence then are passed on to Sam.

[Meisatsu]

Sesshomaru remains to be somebody's brother, however...



Randy, the yet unseen half-brother of Ghostwriter. Randy is a full-blooded youkai in a full-blooded youkai family with absolutely NO ties to any humans — except the technicality that Ghostwriter's soul is human.

The mechanical villain Skulker is quite manifest in the sixth member of the Shichinintai, Ginkotsu.



The Shichinintai leader, Bankotsu, is manifest to a quite a degree in Vlad Plasmius, who employs and thus outranks Skulker.



Vlad Masters in and of himself is a reincarnation of castle guard Setsuna Takemaru, who was deeply in love and in lust with Princess Izayoi.



As before, without a youkai spirit, Takemaru realized "what a fool [he'd] been"...



As with Cad, and Sesshomaru, karmic justice has been done in regards to the misogynist Jakotsu...



Paulina really loves Danny Phantom -- though at current, and outside of RT, she doesn't realize how much she loves Danny Fenton, too.

[Meisatsu]

Somewhat predictably, though shockingly...




...yes, the seemingly dull-witted Jack Fenton is actually the reincarnation of Inuyasha's father, the mysterious but powerful Inu no Taisho. Jack's penchant for invention likely stems from Inu no Taisho's will to protect his human bride, and relinquishing two of his fangs for swords of this purpose. This is most evident in the Specter Deflector, and its presentation.



Thereby, naturally...




Princess Izayoi was reincarnated as Madeline "Maddie".

Deep down, while he was reincarnated as a full human, Inu no Taisho still bore a youkai soul. Therefore, he still had a strong connection with the Youkai no Sekai (or Youkaikai for short) "Youkai World", and desired to breach it once more. This was his primary interest in "ghosts"; Izayoi pursued the interest for Inu no Taisho's sake. She did however say that people in this life might become suspicious of a human so curious to make contact with the youkai world. To that, Inu no Taisho said that he and she would use the guise of being youkai taijiya as their excuse for building a youkai portal and being so taken with the creatures.



In addition, Inu no Taisho was also fond of many foods, one such being fudge, because they reminded him of Izayoi, and the meals they had together.

[Meisatsu]

In another stroke of brilliant karmic irony...





...yes, as it turns out, the seemingly harmless Box Ghost is actually the reincarnation of the uber-powerful and ultra-bad Naraku.




Naraku's form was the result of the merging of the body and corrupted soul of the bandit Onigumo with many, many youkai -- led by a spider youkai who was the reincarnation of the brawly and bitter Overcat, who'd sworn revenge for his defeat at Underdog's hands.




Bad things come in threes...



Overcat's three feline minions were reincarnated as the three spirits of the Monkey God, who were in turn reincarnated in the three vultures which Vlad indentured.

[Meisatsu]

Kohaku was also reincarnated...



Ghostwriter may have done some cruel things, however, that was only because he was "not of right mind"; tormented by his destroyed work. He began and ended up as simply an innocent writer dedicated to his craft.

He IS the walrus! The chummy Kwan is the reincarnation of Chumley...




Nasally and snarky, Pointdexter is the reincarnation of Tennessee Tuxedo...

[Meisatsu]

Jealous and prideful, there is no question Ember is the reincarnation of the "Fiesty Bird-Wielding" Princess Abi.



Further proof lies with Abi-hime's mother, the bird youkai, Tekkei...



Remember when I tried to kick your ass? Danny and Ember have duked it out before...



Born as nothing more than a minor offshoot, Danni is, in body, the reincarnation of the softspoken Kanna, though in mind and speech, she is the reincarnation of the much more emotive Shiori...



...and before that, she was entirely incarnate in the malevolent chiseler, Tap-Tap...



Penelope Spectra's short but uber-loyal assistant Bertrand is the reincarnation of the uber-loyal and spunky Jaken...

[Meisatsu]

In a close twist, Kikyou's sister Kaede is actually reincarnated as her grandmother...




With Naraku being reborn as The Box Ghost, who then, was reincarnated as the towering Pariah? Why, none other than Naraku's proud, nasty little offshoot, Hakudoshi...




And what became of Kagura, who at the end of her life became the formless wind?




Well-versed in his trade, the wise and sometimes wry Lancer is the reincarnation of the swordsmith Toutousai...

[Meisatsu]

The simple-minded but amiable Klemper is the reincarnation of the peaceful Jinenji...




Very young but very defiant, Youngblood is the reincarnation of the Mountain Cat youkai, Bunza...



The two nameless geek girls are the reincarnation of the two airheaded mikos, Momoji and Botan...



Personality-shifting Eliott is a reincarnation of the shape-shifting racoon-dog, Hachiemon, or Hachi for short...

[Meisatsu]

Inu no Taisho's old enemy, Ryuukotsusei, was reincarnated as well. After being torn to pieces by the Bakuryuuha, he and his dragon body were reborn separately -- as Johnny 13 and his Shadow...




An obscure enemy who called upon lessers to do his bidding, Ringmaster Freakshow was, in a former life, the villain called Batty Man. Later, he was reincarnated as the piggish boar, Chokyuukai, who enslaved dozens of girls to be his mate. In his latest incarnation, his persona combined both chauvinism with greed...



Princess Dora Mattingly is the reincarnation of the Black Miko Tsubaki -- her grudge against Kikyou finally laid to rest...for the most part.



His boozing ways laid to rest, Miroku's old master, monk Muushin-sama, was reincarnated as Mr. Damon Gray.

[Meisatsu]

Walker is the reincarnation of Gatenmaru, the rigid, pallid boss of a band of thieves. It's ironic he ended up as the warden of a youkai prison. Not much else to say -- though he did confiscate Tessaiga, and later, Danny's father's anniversary gift.




Harmless at first glance, but bizarrely terrifying once confronted, the Lunch Lady is the reincarnation of the female oni which possessed the Kannon Scroll. Technically, she began her feudal reign of terror using food: salamander eggs, which were forced down the throats of village women, turning them into a mindless and often gravity-defying, ragdoll attack squad.




In her previous life, Desiree was a princess herself, named Sara, who loved the flute -- and Sesshomaru. Her last dying wish was to woo Sesshy. Lifted from her deathbed by a strange billow of jyaki, she became a fierce warrior, seeking to do whatever it took to win the terrifying nobleman's heart.




Greedy, lustful, and not one to care about his own flesh-and-blood, Prince Aragon is the reincarnation of the same-minded elderly bat youkai, Taigokumaru.

[Meisatsu]

As the persistent and ever-hopeful King Kuranosuke said to Sango before, "I waited six years. No matter how much longer I wait, my feelings won't change." As fate would have it, he was reincarnated as the geeky but doggedly determined Nathan, who still seeks Valerie's heart at every turn.




The tough little "Old Exorcist Lady", who was hardly discouraged and always liked keeping everyone on their toes, was reincarnated as the eager, headstrong and often curt Coach Tetslaff.




Bearing the power and responsibility for the well-being of her small community, Principal Ishiyama is the reincarnation of Shujin (the Water Goddess).




Weighted with the task of managing the political duties of his small community, the Mayor is the reincarnation of Nanushi (the Village Leader).

[Meisatsu]

Star is the reincarnation of Nazuna, the young girl who professed to hate youkai, though realized the error of her generalization after Inuyasha defeated the spider youkai.



The Fright Knight is a reincarnation of Goshinki, Naraku's third offshoot.



The Soul Shredder is a reincarnation of Toukijin, the sword made from his own teeth by the disturbed smith, Kaijinbou...



...but...



It is ALSO a reincarnation of Tenseiga, the second sword forged by Toutousai from Inu no Taisho's fangs. The Soul Shredder's main ability as witnessed thus far is its power to send those it touches to "a dimension where [one's] worst fears come to life", or in essence, Hell. This comes from Tenseiga's Meidou-Zangetsuha technique --



The jolly and quite helpful Dairy King is the reincarnation of the wise and jovial Sarugami-sama (Monkey God).