Commander Shadowsun

Commander Shadowsun is a leader of the Tau Fire Warriors.

Official Canon
Named O'Shasera, Shadowsun was a student of Commander Puretide much like Commander Farsight was. Both fought together in the Damocles Crusade, but when the war ended, the students of Puretide were to go on a rotating vigil as one would be on active duty while the others would remain in stasis.

Shadowsun was awoken early because Farsight had left the Tau empire and they needed her to repel an Ork invasion. She was also given the order to capture Farsight should she ever find him. However, she found that they were forced to work together to repel another Imperial assault, though they remained on bad terms.

TTS Canon
The Emperor claimed to have found a recording revealing the true reason why Kitten hated the Tau; it was because Shadowsun broke up with him to fight for the Greater Good, and because every time she was near him it made her forehead sore.

Kitten vehemently denied ever dating Shadowsun, declaring "THIS IS NOT FUCKING CANON!" Despite this, he was also taunted by Magnus claiming that they were dating. The Lockwarden also confirmed that they were dating when Kitten had previously came to him after the breakup.

Trivia

 * In the fan-made Warhammer Idol series by Stringstorm, Shadowsun's debut song "Kitty" seems to heavily reference her supposed relationship with Kitten in the TTS universe.