Dragon Girl went on a quest to find her soul mate, the MC, but when she finally found him, he was already engaged to her 3 best friends. She got upset and almost killed her friends in jelousy since she temperaroly lost her mind with jelousy, then ran away, and MC felt bad so he went after her.
I wouldn't call him the leader leader. More like the defacto-leader whenever the S Rank girl who's the MC's childhood friend isn't around, which she seems to be a lot. Why's she even in this team at this point?
In her debut chapter, it was revealed that she was like this when they were kids. And with the info this chapter that it's not a curse, which she initially thought when she realized she can't be honest with her feelings, it's obviously something mental preventing her from speaking her mind.
Yes, she is a summon in a world where he's the only one with a summon so far. There is very little reason for anyone else to believe him if he tried to tell them she's not a real person or child who he's forcing to fight for him. But I will concede the point that the words he say to himself...
That's not why he doesn't let her fight. She mainly uses her skills to fight. But her skills are so overpowered, that the slimes he usually fights are completely eviscerated. Magic stones and all. If the magic stones are destroyed, that would make a dungeon crawl pointless since he wouldn't be...