Caliban is an upcoming Warframe that is a combination of Orokin and Sentient technology. A warrior built from a twisted combination of parts from both races, their destructive potential is immense. Their Sentient roots are enforced by their passive, as it can resist damages types as it takes more hits, not just for itself but also for the squad.
There is no fixed release date for Caliban as yet, but he will be released with a signature weapon later this year. It is pretty safe to assume, giving the Sentient connection, that Caliban will be released in conjunction with The New War, and will almost certainly have a quest that ties into the new narrative.
The New War will see the Sentients descending upon the Origin System like a swarm of locusts, tearing everything you know about the game apart. Grineer, Corpus, and Tenno will ally be victims of the massive Sentient fleets, and you may find yourself making friends in strange places when the new content arrives.
The New War will also introduce new playable portions of quests for Grineer and Corpus heroes, and fan-favorite Teshin, who will emerge from the Conclave to inflict punishment upon the Sentient invaders.