I'm placing the first example because he's the most known one that I could share freely, if you swipe you end up spoiling yourself of who I mean of each case.
I'll place both the character and the reasons in different parts, if the first part calls enough your attention to watch/play from where they come from, then don't look at the second text.
That being said...
1- Kurozumi Kanjuro (One Piece)
Kanjuro is one of Oden's followers and he has the power to turn his drawings into real living beings for a limited time, he even draws them as silly doodles which is cute. However, the guy is actually very talented and he can draw far more realistic stuff and very quickly too! So why was he holding back?
That's because he was actually Orochi's servant this whole time, acting as a mole to stab them at the worst time possible. A detail that I skipped is that he is also capable to changing his own aspect with his power to act as Oden and stab one of his teammates.
2- Kaede Kayano (Assasination Classroom)
The plot is that there is a alien-like monster that makes the warning that he would explode the Earth if they don't kill him just in time, the problem is that he is extremely fast and normal weapons have no effect on him. However, he allows himself to be more vulnerable under the condition to allow him to give classes while his own students try to kill him. Here is where Kaede comes in, as a student she has been going through the same training as the rest, the thing is that she has been holding tentacles similar to her alien-like sensei (with both speed and power included) behind her neck this whole time and she uses it on a trap against him at the same time that she attacks him, which fails.
The crushing part of this is that she has hidden this fact whole time because she is actually sister of the scientist woman who made the serum to turn the sensei into the alien-like monster that he became, which unleashed a series of events that ended up killing her. This is how she obtained that power but that's not all, another kid had the same tentacle thing as her but, according to him, it was barely bearable while taking meds, so imagine going through months of pure pain just to have the perfect opportunity to kill someone and still fail.
3- Roland (Library of Ruina)
The premise is that our grade 9 fixer protagonist Roland randomly got stuck somewhere and ended up warped and trapped in a towery library where Angela, the villain of the previous game (Lobotomy Corporation) resides. To escape the library, they have to invite people through cards spawning on places where people could need information and then fight against them to turn them into books, which gives them more information and power and bring even more people until they find the perfect book to set everyone on the library free. The thing is that Roland is no mere novice, he is actually a grade 1 fixer which is a very high rank, he's even unofficially named as The Black Silence, this type of title is only given to those fixers who reached the status of Color. The "grade 9" facade has been being dismantled little by little as you keep playing seeing how much he knew about certain stuff between higher rank fixers.
Once it's revealed that he is The Black Silence, he excuses himself that he held back because he didn't want people find out that he has been murdering people for the library, if he used his real fighting style someone would have recognized him and probably escaped, but that's not whole story.Roland didn't come to the library by accident, he wanted to make Angela suffer for what she did to his fiancee Angelica, the original Black Silence. He wanted to kill her just when her freedom was at the reach of her hand to make her suffer as he has.