The Sentry 459 is one of my favorite Marvel characters. He's a robot sure (we'll see about that actually) and he's not appeared in all that many comics, but there's always been something about that implacable mug which I found fascinating. The Sentry debuts (as do the Kree technically) in the pages of Fantastic Four #64 by Stan Lee and Jack Kirby with Joe Sinnott inks.
Simply put, while three of the four members of the Fab 4 plan a little vacation on an remote and isolated island an archeologist and his assistant uncover a vast hidden outpost which they speculate must be of alien origin. At the same time they activate the Sentry a robot (though he does suggest he's more than that when he refers to having been "bred" for his task) who has been on Earth longer than anyone could know. He captures the two humans just in time to deal with the Mr. Fantastic, Invisible Girl, and the Thing as their aircraft is intercepted by the force field the Sentry has erected around the island. A battle rages and it takes the timely arrival of the Human Torch by means of the Inhuman dog Lockjaw to put down the alien Sentry. But that activates the island's self destruct mechanisms and the FF and the two humans they save at the last minute are just barely able to survive as the island disappears, the Sentry along with it.
|(Alternate artwork done for this issue. The replacement is much better and more dramatic.)|
Some months later though we get to see the other side of the Sentry's ages-long mission as we descend into the deep past of the Inhumans who it turns out are an experiment conducted by the Kree on human genetics. The Sentry is left to check on this running experiment among other things. In the pages of Thor #147 in the second installment of a series which replaced the legendary Tales of Asgard we see Sentry 459 perform his duties by visiting the Inhumans soon after their leader Randac has become the first to experience the Terrigen Mists, which eventually gave all Inhumans their sundry abilties and skills.
Here is that story.
Here the Sentry is bright green as opposed to the purple and blue in the FF. We'll discover that quite a bit about the inimitable Sentry 459 is mutable.
The origin of the Inhumans (which is pure Jack Kirby) is remarkably similar to the back story he develops for the Eternals a decade later when he returns to Marvel after his famous DC hiatus. The fusion of the Kree and Inhumans, both indisputable Kirby creations give a depth and luster to the then still young Marvel deep history.
Next the Kree retaliate when comes Ronan the Accuser.