Qalytrix

Qalytrix is former royalty of the Qixs people, an insectoid-aquatic people who live on a once-peaceful planet that, despite its heavy technological advancement, chose to never leave the stars. The Qixs remained on their home planet, and became very suspicious of outsiders, despite being outwardly kind to them. The Catalyst spends a few Earth-months on Qixs, silently studying their kind to figure out why they don't make use of the impressive technology they've developed for themselves. The Catalyst finds out when, one day, the leader of the Qixs, young and new to the role, approaches them, thinking Oh! An outsider! I need advice and they don't know anything, better consult them! Qalytrix explains that the planet is running out of water due to the large amounts Qixs naturally require to live, and that they have replication technology that can save the Qixs, but zher royal advisors refuse to allow zher to use the technology. The Catalyst decides to bond with zher and figure out What Is Going On, and eventually they find out one night when Qalytrix activates an ancient replication device - it turns out that the technology makes one half of the population go mad with rage (half of the Qixs have predominantly aquatic features and the other half have predominantly antlike features, with Qalytrix being a rare Qixs with both features. This is because of a relatively visitation from and subsequent mating with an unknown insectoid species early in Qixs historical period.)

The technology runs on chemicals that, when released into the air, activate a permanent rage within the insectoid Qixs that can never be fully turned off once awakened. It affects Qalytrix, but less, as zhe is only about 25% insectoid; this results in Qalytrix being mostly peaceful but experiencing moments of extreme fury on occassion. The Catalyst helps Qalytrix destroy the source, but ultimately they and Qalytrix must leave; the Catalyst after feeling like they've been in one place for too long, and Qalytrix out of shame for what zhe did to zher people in an effort to save them. The Catalyst promises that they'll find a solution to the Qixs ecological problems during their travels together in the Catalyst's TARDIS.

In some timelines, Qalytrix travels with the Doctor, in which case the Doctor replaces the Catalyst in the backstory.