Rahn
Playstyle: Movement manipulation via magic spells.
Feat: Magic spells gain +2 RNG and boosted attack and damage rolls.
Gimmick: Move your opponent's army around into favorable positions for you.
Units: Phoenix, Discordia, Battle Mages, Magisters
Rahn is all about the Feat turn. When he pops Arcane Alignment, every magic user in his army gets +2 RNG to their attacks and they're automatically boosted. That means more accuracy, more damage and a lot of rearrangement and death for the enemy army. This is why most of Rahn's lists are based off of arc nodes, tons of Battle Mages and Magisters. Obviously, the Phoenix offers the heaviest arc platform at a fairly affordable price and is Rahn's best friend. The newer Discordia works very well with Houseguard Halberdiers because a reforming Shield Wall under Discordia's Imprint is just insane. On feat turn, the Battle Mages spring into action to open up charge lanes and other opportunities via push and pull. The army is very capable of pulling an enemy caster out into the open and to his death. Rahn's spellpool is also fantastic: Telekensis moves things around and turns targets around for backswings, Polarity Shield keeps your Halberds safe from charges and Force Hammer is a POW12 slam that can knock things down. If any of your arc nodes get in trouble, Force Blast pushes enemy models back and the rest of your army can charge them afterwards for a quick kill.
Kaelyssa
Playstyle: A very solid, balanced warcaster that has plenty of tricks to throw around.
Feat: Everything in her control range gains stealth and cannot be charged.
Gimmick: Allowing units to shoot through walls or killing the warcaster with Backlash.
Units: Hydra, Mage Hunter Strike Force, Sentinels, anything really
Since Kaelyssa is one of the most balanced casters for Retribution, she can run with pretty much anything. I've seen her do well with Dawnguard units, Houseguard units or Mage Hunter units. Kaelyssa likes running with two full units of MHSF w/ Commanders just because she can. The best thing about her is her ability to give Phantom Hunter to one of her models. The Hydra is a fantastic choice to give this to because he can shoot a POW15 shot from 15" through a wall and into your warcaster's face. Her feat denies many opponents from getting the first charge and stealth keeps the army safe from shooting. To sum it up, her feat is mainly a defensive one but can also be used to get your units into position. Safety from both melee and ranged threat allows units like Sentinels to move into charge range next turn and force the opposing army into retreat. Mage Hunter Strike Force(s) also add a great deal of threat not because of their stealth, pathfinder, AD, Phantom Seeker and 'Jack Hunter abilities (lol), but because any jacks with low DEF also fear Kaelyssa's Backlash. If you think your warcaster is safe from damage because of their obscenely high DEF and ARM, Kaelyssa will just backlash a low DEF jack and go to town with her ranged units. There's really nothing that Kaelyssa can't do and that's what makes her awesome.