Jen’lig is a thief, which is unique for her race. Due to her race abilities she is also a very useful melee fighter in her own rights with her silver sword. Her abilities are counteracted that she is very hard to control by the player. She is in fact a githyanki plane walker who has learned to detect traps and open locks but there is no way for her to have learned a Primary plane's thief behavior in combat. Yes, she swallows potions like mad if you give them to her, especially in her fighting mood (githyanki fight Berserker style and never use ranged weapons - they want to look their dying enemy straight in the eye.).
You will hear Jen'lig comment on this herself (...use psychic bonds to give orders in battle...) You cannot, so she makes her own decisions. While you learned your battle skills by killing some rats in a warehouse, she has fought mind-flayers, drow, beholders and worse. She has commanded a githyanki ship on the Astral Plane; she is not your thrall on the battlefield.
Jen'lig is installed as part of the Sandrah package, but she is a separate NPC. You need Sandrah in the party to recruit her and you need Sandrah in ToB if you want to meet Jen'lig again, otherwise both are largely independent (although Jen'lig will sooner or later leave you for being an incompetent leader if you kick Sandrah out). You do not need Jen'lig to complete Sandrah's quests, her own quest in BG1 is completed there and independent of Sandrah's quests
In summary, you can play Sandrah without Jen'lig but not vice versa
If you kick her out you cannot recruit her back - she will reappear at the Sarevok showdown but not rejoin you (in her eyes you have disqualified as *commander* - true githyanki logic.)
Jen'lig does not care at all about PC's reputation, why should a githyanki care about concepts of the prime? She evaluates the PC according to the mayor battles (s)he wins and the progress (s)he makes in the quest.