To get Kogasa you talk to her, fight a squirrel holding her umbrella who has a lv 3 boss marker on the map, then take the umbrella to Nitori with the Ingredient items Insect Husk and some sort of leaf thing which are both found in treasure chests somewhere and she'll repair it for some money, then you take it to Kogasa and she joins.
For Youmu you talk to her, get an event where you find her sword, then go back to the original location and fight her.
also since I was curious and Regalstar didn't do them, some sloppy translations of Kogasa/Youmu's stuff:
kogasa's spells:
Karakasa Surprising Flash: fear, impact, and small damage to all enemies, nature elemental
A Rainy Night's Ghost Story: single target, inflicts fear and lowers mind, vacuum/dark/whatever elemental
Lightly Falling Large Raindrops: row targeting, cold elemental
kogasa's skills:
-LP Boost (2 points per level, max lv 5)
-Attack Boost (2 points, max lv 5)
-Evasion Boost (2 points, max lv 5)
-all attacks inflict fear, attacks that inflict fear already have their effect increased (10 points, max lv 1)
-regenerate HP on focus, and regenerate HP at double speed when in back row (5 points, max lv 1)
-not sure on this one, reduces damage from specific types of enemies depending on how many type of that enemy there are as well as decreasing the status resistance of enemies on attack or something (15 points, max lv 1)
-restores <skill level> mp every time kogasa takes damage (5 points, max lv 2)
-restores <skill level> mp every time kogasa attacks an enemy affected by fear, and increases all her stats by <skill level> x 10% (6 points, max lv 2)
-Motivated Heart (5 points, max lv 2)
-Hands-on Experience (5 points, max lv 2)
youmu's spells:
seem to be pretty much the exact same as the last game. The last two are missing descriptions so they might have new effects or whatever.
youmu's skills:
-HP Boost (2 points, max lv 5)
-Attack Boost (2 points, max lv 5)
-Defense Boost (2 points, max lv 5)
-Restores 12% of max HP every time Youmu takes action (10 points, max lv 1)
-Bonus damage to ghosts, doesn't stack with additional skill-holders (5 points, max lv 2)
-Whenever Youmu's attack/mag is debuffed, it's changed to a buff and damage is calculated (I assume this just means she takes regular damage if the thing that debuffs her also does damage) (7 points, max lv 1)
-Increases MP regained by focus by <skill level>*2 (4 points, max lv 3)
-Whenever Youmu takes a turn and her MP is full, all stats boosted by <skill level>*6 (5 points, max lv 2)
-When HP is less than (20 + <skill level>*2)%, increases all stats by 25% (5 points, max lv 2)
-Motivated Heart (5 points, max lv 2)
-Hands-on Experience (5 points, max lv 2)
Pity ITH doesn't work properly on the game, it would be much easier to deal with if that was the case.