I can take Ceria and write another story that she appears in. If I wanted to make reference to any aspect of the TiTS setting, the rights get murkier. If I wrote a sidestory and published it free to view, sure, I don't think Fen's going to go giving anyone any trouble over it. If I wrote a sidestory and monetized it, he's probably within his rights to contest it.
The practical solution is don't use things from TiTS in other things. Don't take your character and use them in another story, make another character who's suited for that story. If you want to take your waifu and use her in a oneshot D&D game, sure, but if you're writing a new Super Waifu Simulator game, don't take a TiTS character you own and shove them in there. This isn't about licenses, it's just about not being lazy.