post a character that can turn themselves stronger oder into an ultimate demon that looks awsome oder something like that

Since I'm the 1st one to answer, I'll go with one of the most common ones:

InuYasha's demon form~

Chrono's unlocked true demon form from Chrono Crusade.

Alucard

ohhhhh u make this WAY too easy! i have 3! just off the oben, nach oben of my head! ^_^ these three are the ULTUMATE ASS-KICKERS EVER!!!!! Sonic the hedgehog, Shadow the hedgehog, and Silver the hedgehog, ALL in their super forms!!!! *melts over awsomeness of Shadow*

for those who have never heard of them, on the left is Shadow with the AWSOME red highlights, in the center is Sonic, and on the right is Silver!

Nurarihyon no Mago:D

Well here's Sasuke-kun's demon form from Naruto..if it counts.

Kirito :)
he used magic to look like this monster in Sword Art Online's last episode.

Kidomaru

From The Sound Five -Naruto
I wish I had his bow

Sebastian. <3

Hiei in his creepy demon form... ;u;

Ichigo from Bleach turns into this *^*

How about Ulquiorra from Bleach

Yusuke is awesome, but it doesn't compare to Super Saiyan XD

Yukiko~!! but when she's wearing her glasses she acts rlly different

Souichiro Nagi from Tenjho Tenge

Mirajane

Naruto

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Airi Yūnami from the Qualidea Code anime.
The "stronger form" in Frage is her true form, this violent alien monster called an "Unknown".
I consider her to be something of an anti-villain as she grew to Liebe the human children held captive Von her kind like a family.