- You may be looking for the Gizou who impersonated Lassar.
Brother Lassar was a Krillitane who, under the name Mr Lucas Finch, took over Deffry Vale High School in order to crack the Skasis Paradigm, which would make both him and his brothers "gods". He and his "brothers" ate at least one school child and several of human staff of Deffry Vale. Lassar tempted the Tenth Doctor with the Paradigm, insisting on "the changes that could be made if this power was used for good"; that he could bring back Perganon, Ascinta or the Time Lords. Sarah Jane Smith stopped the Doctor and reminded him to "move forward". The Doctor declined Lassar's offer and ran. Eventually, Lassar's plan was foiled and he was believed to have perished along with the other Krillitanes when K9 Mark III blew up a vat of Krillitane oil, destroying both the Krillitanes and the school. (TV: School Reunion)
It was thought that Lassar had survived the blast, permanently adopted human form and the name Lucas Finch, and served as prosecutor when the Tenth Doctor was put on trial by the Shadow Proclamation. "Finch" was revealed to be Lau'Tan, a Gizou in disguise, hoping to use Lassar's form to manipulate the Doctor, the Krillitanes, and the Shadow Proclamation. (COMIC: Fugitive)
Under the guise of Lucas Finch or as himself, Lassar always came off as confident, calm, collected and soft-spoken, never raising his voice even during his confrontations with the Doctor. He always spoke pleasantly with other people, even his enemies. He expressed shock at learning that there was another Time Lord walking the Earth, later recalling that the Gallifreyans were "ancient, dusty senators" frightened of change and chaos - he showed an altogether layered opinion of the Time Lords by reputation, and was quietly enthralled at the opportunity to meet one. Despite expressing the opinion that the Doctor was starkly different from the rest of his people, he strove to make an ally of the Doctor several times, trying to entice him with the offer of the Time Lords being returned from destruction. He even seemed assured that the Doctor would join them once he discovered their plans.
Lassar showed a vain streak, claiming that he maintained human form because it was a personal favourite, and expressing eccentric pride at Sarah Jane Smith writing a profile about him for the Sunday Times. He showed a rather disturbing sense of humour, remarking at one point before he and the other Krillitanes ate a room full of humans, that they were having an "early lunch". However, behind closed doors he could prove to be as savage and ferocious as any Krillitane, perfectly capable of killing and devouring anyone who got in his way. He showed animalistic fury when K9 entered his battle with the Doctor, screaming intensely at the sight of him, and derisively taunted him when he thought K9 had weakened. Having lost patience with his battle with the Doctor, he shed his value for the Time Lord and allowed his siblings to eat him but to bring Lassar his brain. He expressed cold hard rage when K9 set off vats of Krillitane oil, foiling his schemes once and for all. (TV: School Reunion)
Unlike usual Krillitanes who typically change their physiology with each race they conquer, Lassar chose to adopt a human form, claiming it was a "personal favorite". Even in this form, he possessed a superhuman level of strength, easily punching through a wall without injuring himself, as well as overpowering and devouring his victims within seconds. (TV: School Reunion)