I look at it as T’Challa trusting an outsider enough to give them sensitive info and leadership of his people. Kasper serves a unique function in that way (the foreigner) on top of being one of only a handful of characters that T’Challa has a relatively positive, well developed relationship with. I agree not every character should be main supporting cast but if the limit is at 4 (which feels appropriate) I think he should be on the list with Okoye, Shuri, and whoever is his love interest. With M’Baku, Ramonda, and some equivalent to Zavawari serving as extend cast. I think the character has lots of potential as one of the more prominent BP side characters.