Rem is most frequently used as a healer/support that heals/buffs the party and can pull up allies to keep hitting enemies. With her LD buff, Rem herself is capable of some solid damage while she does this (4 AoE HP attacks after her actions). With her BT & FR upgrades, Rem gets even more support/utility by refilling the party's EX gauge when she acts, preventing KO from normal HP attacks, tons of battery when allies are healed and full HP on top of the more normal BT effects. Her damage is also much stronger as if needed, she can dish out 8 AoE HP attacks thanks to EX/LD + her LD follow-up. However, note that her EX has high turn rate and will jump up which you may not want in Force Time (to extend her BT effect for survival).
Cure (S1) is a free turn with instant turn rate. This will heal and battery the party while granting Rem her Devoted Heart buff. Devoted Heart will boost the party’s attack and grant an HP regen.
Swap Turns (S2) is a free turn with a high turn rate. Rem will swap turns with the targeted party member and battery herself and the target and granting them both generic stat buffs. The party member’s EX gauge is also boosted by 20%.
IMPORTANT: For those who want to use these skills to fill the FR gauge, note she only fills the FR gauge/supercharges the FR HP DMG Bonus at 3/5 efficiency compared to other FE30 dedicated FR gauge fillers like Aerith, Sherlotta, etc.
Siphon Delta (EX) is Rem’s Fast+ EX, taking 1 turn to refresh. Siphon Delta batteries the party before attacking, then healing based on the damage done with excess going to BRV. This also increases Rem’s attack and grants Siphon. Siphon allows Rem to have a free skill use on her next turn. It can also be extended with Sazh, Eiko, Hope or Yuna calls for very unique strategies; this also means her BT phase can be all free skill uses for FR gauge charging.
Will Carried on Twin Blades (BT) is loaded with utility: Party-wide KO Prevention (Zack BT, Ardyn LD, Galuf LD effects), Excess heals turn into BRV, EX refills after Rem acts (Bahamut Megaflare buff effect) and other useful BT effects you're used to.
For damage, she can EX 4x, LD and FR for pure damage if need be and if you entered BT phase with at least 3 turns of BT effect.
You can also just use the BT phase to fill the FR Gauge, but remember the important note stated above!