Regeneration is a thread ability, coming in four separate levels. Regeneration replentishes health for every tick or few seconds.
Regeneration is not naturally given to any player in the game, so to have regeneration, a certain thread has to be worn in order to gain the regeneration.
Whenever the shared HP bar is less than full, the wearer of the thread with this ability will regenerate HP every few seconds, higher levels granting a greater boost in HP per tick. The HP Bar will fill on the side of whoever is wearing the thread; overflow will fill the other side. However, remember it is a shared HP and so it really doesn't matter which part of the bar fills up first. If both characters have the Regeneration ability equipped, the amount of healing will add up.
For example, if Neku heals 160 HP and Shiki heals 120 HP per tick, the amount regenerated would be 280. All Regeneration Threads stack with each other, adding the percent together.
Regeneration has four levels. Level I restores 1% HP/tick, level II 2% HP/tick, level III 3% HP/tick, level IV 4% HP/tick. Regeneration that is "Jinxed" will grant regeneration, but in trade of a negative factor, such as stopping PP gain.
Sis's Pendant - Regeneration IV
Darling Lock - Regeneration IV
Mrs. Claus - Regeneration III
Plumeria Studs - Regeneration III
New Year's Gift - Regeneration III
Lovely One-piece - Regeneration II
Green With Envy - Regeneration II
Plain Shirt - Regeneration I
Mint Choco-chip - Regeneration I
Flaming Knee-highs - Regeneration I
Double-breasted Suit - Jinxed Regeneration
Ribbon-laced Dress - Jinxed Regeneration