Ithea is considered a full grown Leprechaun, fighting alongside Chtholly, Nephren (, Rhantolk, Nopht and may prior as upcoming Leprechauns). From start just her character is know, but some information affecting also the knowledge about Chtholly will be revealed along the way.
SPOILER ahead: in LN vol3/Ep9 of anime it is revealed that she isn't he Ithea that was once. 2 years ago (probably due to her personality) she went under a somewhat similar, although different process as Chtholly is now. (Chtholly wished to come back no matter what.) The results have been an instant takeover of her prev. life, which forced her to lock herself up in her room, stating she won't feel well. Lucky she found a diary, which gave her pretty decent information about the body the new 'Ithea' possesses and her surroundings. Remembering back, the LN references that only those close to her might have noticed a little difference/Chtholly thought that her behavior back then was a bit strange. For Leprechauns to come back alive from a battlefield is rather rare and it is further rare for the encroachment to take place before their 20. Before Chtholly it was not known that they could live on from that point on, but Itheas revelations proofed, that theoretically, it could happen to any Leprechaun.