@djlryalief I don't know why you say like that. I think the manga already clear to tell the situation.
1. Aiko mission is to find Shimizu who missing from the party.
2. Hajime mission is from adventure guild to find noble kid and his adventurer's escort.
3. Aiko just meet with his dead student who actually still alive, she want to have more conversation, they have the same mission to find a missing person, so it is a perfect opportunity.
4. Hajime only agrees to tag along together(because Aiko is so stubborn), Hajime doesn't agree to find Shimizu.