The Shadow Isles are lacking a champion who is not just "evil" in appearance/personality

Just look at them, all evil (from our perspective) and dead. They just lack the variation other factions have.

In the Freljord you have evil witches and warlords, but also someone like Braum or Gragas. Demacia has the cheerful lightbulb of doom who stands in total contrast to the likes of Garen or Fiora. Heck, even Noxus has more personality variation going on than the Isles with . Ionia has quite a lot of personalities, from stoic ninjas, to clerics to batshit crazy psychopaths. And don't get me started about Targon... the aspects and Sol are clearly total opposites in terms of character. And Bilgewater has everything, from crazy pirates to tricksters to the happy tentacle priestress.

But I can understand how this is troublesome: how do you make an undead character that is possibly controlled by the Black Mist, into a "not completly evil" character?

I doubt there will be a single undead that is powerfull enough to directly fight against the Black Mist (apart from Mordekaiser who is Numero Uno.). However, there might be some who simply do not care about what the Black Mist or Mordekaiser want from them.