Mika is character who’s been introduced in the current event: Of Ballads and Brews, harvest and winemaking event in Mondstadt.
In the story, he have little boy like appearance with yellow hair. From his appearance, we know that Mika wield Cryo vision although there’s no details about his weapon.
Mika is member of Eula Recon party as explained by Lisa. Initially coming with Varka expedition in front line but return during Weinlesefest event in Mondstadt to deliver letter to Knight of Favonius.
Hopefully Mika will make more appearance in current event so we will have more insight about him.