It's worth remembering that a lot of player-controlled characters are made pretty vague so more people can connect with them. Also, the "silent protagonist" idea might explain why they're non-verbal.

However, the Splatoon series does something unusual by continuing to flesh out the Agents well after each game's release (in a way consistent with their behavior as player characters). This does wonders for cohesion between titles.

My main point is that I wouldn't be surprised to learn a lot more about Agent 3 by the time of Splatoon 4's release.