You can obtain Lemuré by completing the first mission of "Lemuré acquisition quest" event. The event only gives a total of 240 shards, so you'll have to get the rest from shop/gacha to max her out.
She is a support-based character that features a lot of important skills to help a fight. Time Mage main job has Quicken and Hasteja, while her White Mage sub-job has Esuna, Cure/Cura/Curada/Curaga, and Raise. She's able to deal some decent damage too, but won't really compare to other mage damage dealers since she lacks any MAG increasing ability and any form of penetration.
Her Scholar sub-job is there to help increase casting time through the passive, but using it as sub-job makes her cast even faster because of Law of Speedreading. It also comes with some offensive spells, which is the only way for her to deal Dark damage to enemies.
Her Time Mage can be a little lackluster outside of Quicken without her EX 120 ability, which is why it's quite important to max her out if you plan to use her. Her job lvl 22 counter ability upgrade is also pretty useful in content like PvE against bosses, but might rarely be used in PvP if she often gets one-shotted due to her bad resistances.
Looks like a pretty good support for Duane and Helena, however, I think using her as a haste/ quicken bot and healer primarily to help Duane and Helena would be the most valuable, and in that case, the fact that she is a dark unit means she won't be element chaining unless you run her scholar sub, which means you lose out on heals.
So in other words, you'll get a few dark only bonuses from master abilities and VC's but I suspect Yuna, Aerith, Ayaka, or a proper tank with Helena and Duane would be overall better. Quicken though... pretty tempting.