Who does serenity work with?

Serenity is really only good for debuffing enemies. In that regard, she could probably work in most teams, though I would say it's best to pair her with a more reliable DPS since her damage is very low. A better support would also enhance the team.

She does have 2 arts cards, so she could be viable with saberlot in an arts-crit team: she could provide a few extra stars for saberlot when his skills are on cooldown while also draining enemy NP and sealing their skills. Tamamo would be the best support for this kind of setup. Her NP drain will complement Serenity's, plus she's just the best arts support in general. Bear in mind though that her star generating skills are okay, but not great as far as assassins go.

Well if you don't have a crit star gen and could considering an Arts deck plus star gen of the high rank class skill then yes. I'd pair them up. Lancelot could provide his own stars, but more crit stars isn't bad, right? More than that there would be more arts chain and she could probably spam her NP easier saving Lancelot's ass from an enemy NP when lucky.

Yes but i would use hans for most cases and use serenity vs riders