How come nobody received a level up from killing the fire-breathing wolf boss? Surely Mia herself should be leveling up. Yet it's Arisu, who was doing nothing but delaying the orc boss. And most certainly Nagatsuki should get a few levels at once, if she was still level 1. This whole level up thing sucks noodles.
The manga operates on a party basis, where everyone in the active party gets an equal cut of the experience points. Mia might just need more experience points to level up than what she got from the sub-boss that just got killed, and Nagatsuki not being in the active party could have leveled up without the MC - or us - knowing about it, because they wouldn't have come to the same level-up "instance" as the MC and his party.
Solo Players enter the White Room alone:
Arisu levels up and enters the White Room, picking Spear Arts by herself.
Players can only Party-up at Level 1 or above:
Arisu couldn't form a party with MC because she was Level 0.
Only party members can be seen in the White Room at the same time:
All party members enter the White Room if a Party member levels up.
Party members can select who to accompany them to the White Room:
Accompanying Function allows the player to check/uncheck party members.
Experience points are shared among party members:
MC figures out he needs to kill twice the enemies to level up in a party.
White Room functions as Las Vegas, What happens in the White Room stays in the White Room:
Arisu loses her virginity in the White Room, but her body condition will revert once out of the White Room.