Ralsei is a deuteragonist of Deltarune and, presumably, the "Prince from the Dark" of the Heroes of Light, as foretold in the Prophecy.
Having weaker attack and defense, and with the ability to cast healing spells, Ralsei acts as the party's healer. Until meeting Kris and Susie, Ralsei lived alone in Castle Town, not having any subjects despite his title of prince. It is unclear if the others inhabitants ever existed at some point. Ralsei knew about the Prophecy and waited for Kris and Susie before finally meeting them.
Intelligence:Genius (Has massive knowledge about the Dark Worlds, even one's that they don't belong too. Is aware of what will happen if one doesn't seal the dark fountain and a master magician.
Hearts Donut: Recovers 20 HP in the overworld and 10 HP in battle.
Lancer Cookie: Recovers 5 HP in the overworld and 50 HP in battle.
Revive Mint: Heals a fallen ally to MAX HP. A minty green crystal.
RouxlsRoux: Recovers 50 HP in the overworld and 60 HP in battle.
Spin Cake: Spin Cake is a secret item that can be obtained by bringing the repaired Top Cake to Top Chef. It is only half as effective as the Top Cake, but it can be replenished infinitely by Top Chef should it be consumed or dropped. It restores 80HP to every party member.
Top Cake: The Top Cake is an item that can be obtained from the smith Malius when asked to fix the Broken Cake. The Top Cake restores 160HP to every party member, making it the best healing item available.
CD Bagel: Recovers 80 HP.
Revive Dust: heals all downed allies to 25% HP.
S.POISON: Removes 20 HP in the overworld and slowly drains 60 HP in battle.
Spaghetti Code: Recovers 30 HP.
Butler Juice: Recovers 100 HP.
Tension Bit: Increases Kris's TP by 32%.
Tension Gem: Increases Kris's TP by 50%.
Tension Max: Increases Kris's TP by 100%.
Kris/Susie/Ralsei/Noelle Tea: The tea is a consumable item that comes in a unique flavor for each party member, and has various healing amounts and party responses depending on which tea is given to whom, seen here.
Light Candy: Noelle gives the Light Candy to Susie, if Kris spoke to Noelle about Susie in the Chapter 1 epilogue. Recovers 120 HP.
*Manual: The Manual is an item given to Kris by Ralsei after they complete the Training Dummy fight, or during the dialogue with Ralsei if Kris refuses the tutorial. While used to learn tips and tricks in battles it can also be used to make enemies fall asleep.
Bouquet: Nice flowers.
Glowshard: The Glowshard is an item found hidden inside a hole in ??????. If checked in the inventory, it states its use is unknown. The Glowshard can be used in battle to immediately allow for a Rudinn to be spared. It disappears in the process.
Notable Techniques
FIGHT: Self Explanatory. Once the FIGHT option is selected, a quick-time event occurs. A cursor slides across a meter from right to left. By pressing Z anytime the cursor is within the meter, the party member successfully attacks the enemy; otherwise it results in a miss. Hitting "Z" on the box located on the leftmost area of the meter deals the most damage, which emphasizes proper timing.
SPELLS: Ralsei can casts a variety of spells, the following here:
Heal Prayer: A healing spell that requires TP to use. Can heal himself or others.
Dual Heal: An upgraded Heal Prayer, only accessed after Kris shines their SOUL's light on Ralsei. Requires TP, and heals all of Ralsei's allies.
Hypnosis: Ralsei chants something, making the enemy feel tired.
Pacify: A method used by Ralsei to peacefully end battles against tired enemies by putting them to sleep.
Sing: Ralsei sings a lullaby, putting his target to sleep.
FluffyGuard: Ralsei creates two cotton-like shields that protects his ally from bullets.
ACT: A pacifist method used to make even the most violent of enemies defeated through kindness. Ralsei regularly does a team ACT with Susie or Kris.
ITEM: Used to consume healing items.
SPARE: An enemy can be removed from the battle with this option as they no longer want to fight, if their name is colored yellow, and a star appears beside their name.
DEFEND: Self Explanatory. A party member is prompted to protect themselves during the enemy's turn, reducing damage and gaining TP immediately once commanded.
Other
Standard Tactics: Ralsei's main tactic is to pacify. Their leading move is even pacifying enemies as shown in cutscenes.
Weaknesses: Ralsei is extremely kind to a fault, getting tricked by the likes of the Chaos King believing he truly gave up.