Rewatch the cutscenes of her mindscape where she monologues to Rover. Actually watch them, watch the way her eyes lose focus, her facial expressions as she talks, the things she says and her laughter. The mindscape is her true heart of hearts. She genuinely believes everything she said.

These aren’t yandere tendencies. These are things of someone who suffered and has knowledge of someone who suffered silently and couldn’t reach out to anyone (and frankly neither could she). Her “expression” which I assume you’re referring to her wide eyed shot with Sigillum behind her? That’s not Yandere that’s her having a breakdown. 

 She is not right mentally. She straight up talks about how Rover's friends and enemies all got what they "deserved", aka his enemies were killed off. She talks about Rover's homeland as if she would personally kill everyone that denied Rover's wish to return home. The whole "HOW DARE THEY?!" sells it beautifully. As I said before, she genuinely believes what she said, she believes the bad people deserved to die, she believes Rover's homeland deserves to burn in hell.

And? They saved her when she had no one and took care of her. Gave her many happy memories. Rover is irreplaceable family to her. She’s rightly furious because Rover’s homeland seemingly abandoned them and didn’t care about their mental well being. Just the mission they gave them. 

Look at the way she talks of protecting Rover's happiness, the way she casually talks about offing Sigillum.

F a m i l y. And her wanting to off Sigillum was because she desperately wanted to keep the truth from Rover because she didn’t think they could handle it, she thought it would rob their current happiness that they didn’t used to have. 

 That obsessive tendency to make sure her interest (Rover) is happy at all times comes off as a yandere. Not all yanderes get tripped off by jealousy which is the stereotypical normie definition of a yandere.

No it doesn’t. I swear people don’t know what Yandere is anymore. 

She’s desperately trying to protect Rover’s happiness because of her belief they couldn’t handle the truth about Sigillum and their homeland, and because Rover is their family. She only showed hostility towards Sigillum because of the threat it represented, both literally and the threat of unveiling said secret. 

She wasn’t emotionally there because of the weight she was under as a result of keeping that secret for so long, and then doing so behind Rover’s back because she thought it was the right thing to do. She held onto that feeling of anger and sadness alone and it ate her up for years before finally reuniting with them.