Fasili Mirembe was a high-born Soninke lord, the (great-)nephew and appointed heir of High Lady Abreha Mirembe of Aksum and considered the right hand of Akua Sahelian. He died during Second Liesse.
Fasili is described as a "fair hand at flattery" and having the "stunning good looks" usually seen in high-born Praesi; Akua considered him "everything a noble of Praes should be: ruthless, patient and subtle". Although his military record was slim before the Uncivil Wars, he showed himself to be a talented commander, having been tutored by Asmund of the Dark Teeth Clan, senior tribune of the III Legion, and Lady Taslima Ubid, senior mage of the XI Legion. Unlike most war-minded Praesi aristocrats, he also studied the treatises of Grem One-Eye, the "greatest military mind of their age", even though the author was an orc, and after the war with Summer he was considered Akua's best general. 
He aligned himself with Akua when she was Heiress, presumably in the hope of becoming her Chancellor once she claimed the Tower, and after she explained to him what she considered the true nature of the threat Dread Empress Malicia posed for the society of Praes he stayed with her even when she became the Diabolist.
The Mirembe family have ruled Aksum, one of the seven High Lordships of Praes, for at least a thousand years, and the current High Lady – Fasili's (great-)aunt Abreha – is quite old. Her heir, Fasili, seems to have been at odds with her for quite a while, to the extent that her first reaction to Catherine's threat to cut out his soul was to outright dismiss it; only the possibility that she might give the soul to Black so that he could use it to gain control of Aksum made her hesitate. In the end, she accepted Catherine's demands and Fasili was returned intact; however, within a year he was the former heir of Aksum.
In the Battle of Dead Dawn, Fasili led a troop of twenty-three thousand, mostly consisting of wights, with the intention to crush and turn the eight thousand men sent from the Ankou watch before they could join up with the Fifteenth Legion. Catherine, leading the Fourth, Fifth and Ninth Legions plus Hune's jesha from the Fifteenth, barely reached them in time, bringing the number of defenders up to twenty-two thousand. When his countermeasure against her Name power failed due to the simple trick of having Vivienne Dartwick, the Thief, pose as her while she herself hid among the soldiers, his strategy fell to pieces and he fled back to Liesse. After the battle Hune ranked him as skilled but inferior to most Imperial generals, although with the caveat that the difference was not big enough that it would be likely to make a decisive difference.
During Second Liesse he was commanding his troops from inside Liesse itself but ran afoul of Robber and his cohort, who stole his skeleton key and brought it to Catherine at just the right moment. If he somehow survived that encounter (it is not quite clear), he would have died either by the released Deoraithe souls or by the rampaging wights.