Kryptman

"He was exiled from the Inquisition for being too cruel."

– Rogal Dorn about this madman

Kryptman is an Inquisitor (well, ex-Inquisitor) of the Ordo Xenos who has dealt extensively with the Tyranids. He was not the first human to have contact with them, but he was the first to identify and name them when he investigated the loss of contact with the Tyran research station after its destruction by Hive Fleet Behemoth. He also roused the Imperium to take a stand against the threat. Picture him as a combination of Jurassic Park's Alan Grant (book!Grant, who was quite badass) and, of course, Ellen Ripley from the Alien franchise (obviously, the film "Aliens").

As the Tyrannic Wars continued, Kryptman became one of the foremost researchers into means of halting the Tyranid advance, but his determination to end the threat consumed him, and he came up with a couple of ideas that even his fellow Inquisitors thought were too crazy to implement.

First, he created a cordon by committing Exterminatus on a "moat" of worlds in the path of Hive Fleet Leviathan. The idea was that, as Tyranids tried to get through the cordon, they would starve to death. It did slow Leviathan's advance, but the loss of a dozen planets on one Inquisitor's say-so was too much for the rest of the Inquisition as it was the single biggest loss of life in the Imperium since the Horus Heresy. They told him to cut it out and expelled him from the Inquisition, but he was far enough away that he could not actually be arrested. One school of thought is that Reasonable Inquisitors might do something similar by defending a world, and wait to commit Exterminatus once the world was deemed lost, denying the nids the biomass or even causing a net loss in biomass, while potentially saving the planet to boot.

When he couldn't get everyone to buy in on the cordon plan, Kryptman decided that if the Tyranids could not be destroyed, they could be used. So he used some Genestealers captured during the course of the Exterminatus campaign to lure a splinter fleet into an Ork empire. Unfortunately, the Tyranids failed to wipe out the Orks, and the Orks also failed to wipe out the Tyranids, leading to the Octarius War. It did buy time for the Imperium, but the damn fungi and bugs are both feeding upon the other's aggression and becoming stronger and stronger as they keep chopping each other to bits...

In the end, as more and more Hive Fleets entered the Imperium, Kryptman's cordon idea started to look better and better to the authorities, but Leviathan's attack from underneath the galactic plane proved that such a cordon would have to surround the galaxy in all three dimensions, which might be more than the Imperium could bear.

Right now, Kryptman is working once again with the Deathwatch, trying to figure out what in the Emperor's name is Hive Fleet Tiamet doing in the planet Ziaphoria, where they are building some sort of massive structure...