@KingAccel. Hmmm..... I'd say you could use her sort of like how Diablo is to Rimuru or Lancelot was to Godou if you wish to add her, like a subordinate. Where she becomes a bit fanatical when it comes to Jaune. Very willing to do anything he asks, "ANYTHING", but considering Jaune's personality and theme the likehood of that happening is next to impossible.
Cause her personality and what kind of entity she is, is what would make her difficult to deal with in how you add her. Besides the girls in the harem, and Yoruichi as a teasing friend, who may or may not become a member. Testarossa being a bit of a fanatic who feels that Jaune can do no wrong and that following his every command is her purpose in life. I'd say more but I'm trying to keep it short for now, if you would like to hear more about adding or using her in another story later just ask.
I do like her, though if you add her to the story her being like Diablo and utterly devoted to Jaune would be the way to go, though that's how primordial demons who serve others often act. Though I will say that writing her like that would mean she'd look at her master's harem and say "My lord has too few concubines/etc for a supreme being such as he, It'd be next to impossible finding those worthy enough to join, but it must be done", by which point you may need to add a warning label lol. So maybe not so extreme.
That said Testarossa being a subordinate God who falls in love with Jaune after being defeated by him would be the way to go. But, I wouldn't really replace anyone for her, because she wouldn't care who else got close to Jaune, because ultimately they would be the same as her in serving her king. Only viewing the situation as being expected of a being as great as him. Whether he took her as his lover or just a servant would be relevant since she would put all her heart into it. That's how I would see it anyway.
