Also this form had description that one head is malevolent and the other is benevolent. Judging from the fact that the other head didn't try to eat allies and has support skills it's a benevolent head, how lucky for our MC to get a malevolent head role.
Cause otherwise it would be very hard to execute a rescue.
Given the male-female dichotomy, it makes some sense. The male head is the offensive one, the female head has support defensive abilities. Really lucky that the head capable of leveling cities is more rational (probably)
The main character really needs to start trying to talk using words. He possesses level 3 skill in the dominant human language. He has a mouth, teeth, tongue, and lungs to expel air. Literally experiment to make sounds. It makes no sense that he shouldn't be able to do this in DRAGON form. But the author pretends like it's only something he can do while being a human. There is nothing preventing him from trying, other than the author clearly want's communication issues to be a main storyline and humor source. But they provide no justification for it, like a curse that would make him silent, or something.
I normally don't mind silly, and one of my favorite parts about Illusia is how much of a dunderhead he can be. It's been so long I can't remember any specifics, but if I stretch my memory as far as it can be and try to reconstruct my specific feelings based on vaguely remembered distaste and annoyance, partner came off as needlessly difficult or contrarian, and that wore out its welcome quickly. I could be completely off base and making shit up, but that's usually the only thing that so thoroughly kills my interest in a short period.
At first she was being difficult because she felt the MC always sticks their noses in others problems and then choosed the most annoying path to resolver everything, like with the Tribe, but later after Treant, Allo and Nightmare get into the picture she softens up a lot, she is still a little bossy and becomes a food junky (She wants to eat EVERYTHING they encounter, being flowers, monsters, even minerals and rocks!) but is on board with almost everything Irushia does, and even take his feelings into considerations, when he had to kill an army of hundreds to defend the village, after starting the massacre Partner offers to take control of the body and the burden herself so Irushia doesn't feel bad about it, he rejected the idea because it was his desition and he have to live whith the consecuences
To me personally she was a highlight of the first half of the story on how the MC can influence good in others and his good nature brings the better in his companions
When partner choose to sacrifice hershelf for the sake of Irushia and uses Sacrifice to slow Ruin and defeat him, the talk they had prior showed that Partner really saw Irushia as a Brother and loved him (Not in a romantic way) and their companions a lot, even saying she was greateful to be born as Irushias second head and partner
I recommend reading the special chapter of volume 5 of the light novel title: "Life as a Two-Headed Dragon - Sleeping Edition" after reading that I really started to like partner a lot