Because Nahida needed to do some business in Inazuma, Wanderer was forced to come along. That was where he met you, Makoto's child. The business Nahida was called here to was to take care of you while Ei attends to some other business where you aren't allowed to.

Wanderer and Nahida were back in Sumeru along with you.

"So, you're my cousin?" Wanderer interrogated you suspiciously.