“Sentinel” was part of Marvel’s “Tsunami” brand of titles, which all skewed to younger audiences and were drawn manga style. This first arc was intended to stand alone from any of the other X-titles, so it’s set in Wisconsin where a high schooler named Juston Seyfert finds an old Sentinel robot in a junkyard. Seyfert is a computer genius and is able to rebuild and reprogram the Sentinel to be his friend.
Eventually, the government finds him and tries to reclaim the robot. That’s the source of the action-based conflict for most of the book. The through-line in terms of story is Juston working through some personal issues with his family.
I’m not covering in detail because this is really an all-ages book that, while it is in the 616, isn’t terribly important (or interesting). It’s a solid book for a 12-year old—and I’d recommend it for young readers.