Job 16: returning assist
Assuming that the assist doesn't have to be from a series that has a playable character in this game...
Light Yagami and Ryuk (Death Note)
Light Yagami is a brilliant Japanese teenager who discovers a notebook used by the gods of death to kill humans. He has a strong sense of justice, but the power that the Death Note gives him warps it, leading him to believe that the world is rotten and he needs to use the Death Note to purge it of evil people so that it can be fixed, declaring himself "the god of the new world".
Together with Ryuk, the god of death that dropped the notebook, he enters an elaborate cat-and-mouse game with law enforcement agencies, a woman who's madly in love with him, and a mysterious detective only known by the letter L.
Death Note is a very unusual Shonen Jump series, and Light himself isn't much of a fighter - I'm pretty sure he only threw a single punch over the course of the series. His main form of fighting consists of elaborate mind games and, of course, the Death Note - once he knows your name, he just has to write it down, and then you have 40 seconds until you die of a heart attack. (though he can get much more creative and detailed, which he figures out as he tests the notebook's limits)
In Jump Ultimate Stars, his 2-koma has you select a target, and after a long delay, they're hit with the Judgement/Doom side effect if a falling apple hits them. This starts a ten second countdown that kills the opponent once it ends, though the apple is easily avoidable in most situations. The 3-koma version does this to everybody. However, as there's no touch screen to select people, in this game, the 2-koma could automatically target whoever has the most damage.
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For reference, here's a guide to the effect of every single support character in Jump Ultimate Stars, so you can describe what a character does even if you haven't played the game:
For Jump Ultimate Stars on the DS, Support FAQ by shinyspark.
gamefaqs.gamespot.com
I can't find a similar one for Jump Super Stars, though there's a lot of overlap from what I've seen; the main exceptions are the Naruto characters, where they hadn't done much post-time-skip at that point, so they weren't carried over. (at least, that's the only explanation I can think of for why Lee, Hinata, Neji, and Shikamaru didn't even get help koma, let alone support koma)