Hilly is a Tenth Generation AI Hologrammatic computer on board a version of Red Dwarf where the crew are female. played by Hattie Hayridge. She is a female version of Holly. Hilly makes her first appearance in the episode finale of Series II, "Parallel Universe".
This is Hilly's one and only appearance, however the actress who played her returned to play female Holly in Series III-V. Norman Lovett reprised the role of Holly when the character returned in the Series VII finale "Nanarchy" and for Series VIII.
The effect on Holly was explained in the Series III opening text crawl in "Backwards":
Meanwhile, Holly, the increasingly erratic computer, performs a head sex change operation on himself. He bases his new face on Hilly, a female computer with whom he'd once fallen madly in love.
Her behaviour is likely to be identical to her alter-ego Holly, since their histories are parallel. She speaks in a London accent and appears to find her counterpart attractive. The two have a brief love affair.