The easiest way to give a Persona new stats and abilities in Persona 5 Royal is to give it experience and level it up. However, Personas gain levels slower than Joker and the Phantom Thieves because the game wants players to constantly make new Personas and raise confidant ranks to get the Arcana Burst bonus.
Fortunately, players can also improve Personas by using Lockdowns and incense. In fact, these methods work equally well on Personas that have reached level 99 and can't gain any more stats or abilities from experience. By using these methods, players can create a truly unstoppable Persona for the late-game of Persona 5 Royal or for a New Game Plus run.
How to Get and Use Lockdown
The Lockdown ability is something players can access once they reach rank 3 of the Strength confidant. It shows up in the main menu of the Velvet Room as "Train Personas."
With this ability, players can choose a Persona currently in Joker's stock and put it in Lockdown. When a Persona is in Lockdown, players can't use it in battle, can't give it experience with the Gallows or by claiming the same Persona type as a mask, and can't fuse the Persona with anything else. However, a Persona in Lockdown does count for the same-Arcana bonus when meeting up with confidants.
For each free day players leave their Persona in Lockdown, the Persona will gain one stat point. In addition, a Persona can learn a passive skill that helps them avoid an element type. Personas with weaknesses will most likely learn skills that protect against those weaknesses, but if a Persona has no weakness (or already has a passive skill that protects them), the skill they learn will be random.
The speed with which a Persona learns this skill depends on Joker's rank with the confidant of the same Arcana. At rank 0 the Persona will learn the skill in 5 days (not counting story days when the Velvet Room is inaccessible), and at rank 10 the Persona will gain the skill in 2 days.
The type of passive skill partly depends on the Persona's level. Up to level 33, a Persona will only learn Dodge or Evade skills, then Resist at level 34, Null at 53, Repel at 63, and Drain at 75. A high-level Persona has a chance of learning a less powerful skill, but a low-level Persona has no chance of learning a high-power skill. In addition, the skill learned is set when players put the Persona in Lockdown, not when they pull it out. Players will need to reload and replay several days if they want to fish for a good skill.
However, there's also a chance that Joker will lose the Persona in Lockdown. After 10 days (not counting story days again), the Persona will drop out of Joker's stock and lose all the skills and attribute points it would have gained. Fortunately, the Twin Wardens will call Joker to remind him about the Persona before this happens, and it costs nothing to visit the Velvet Room and take the Persona out of Lockdown. In fact, players can put the Persona back into Lockdown to spend another 10 days gaining stats.
How to Get and Use Incense
While a Persona is in Lockdown, players can light some incense to further improve its stat gains. Regular Incense boosts one stat by 1, Ambergris boosts one stat by 2, Nirvana boosts one stat by 3, Musk boosts two stats by 1, and Rasta Sandalwood boosts St, Ma, and Ag by 1. Each incense needs two days to take effect, and after that players can light another incense to get another bonus.
In addition, a Fusion Alarm doubles the effect of incense if it's active when players use it. Joker can start a Fusion Alarm even on days when he doesn't go to the Metaverse by asking Chihaya to perform a Celestial Reading.
Players can find incense in several locations throughout the game. The Mantra Ganda shop in Kichijoji sells Incense as soon as the district hub unlocks, Musk starting in July, and Rasta Sandalwood starting in October. Jose in Mementos sells Ambergris and then Nirvana incense, and he refreshes and improves his inventory every time players enter a new area.
By combining Lockdowns and incense, it's possible to eventually give a Persona 99 in every stat, making it all but unstoppable. It may take until late into a New Game Plus run to reach this point, but a Persona like that will be a big help when fighting the bonus bosses that also unlock during a New Game Plus.
Persona 5 Royal is available now on PC, PS4, PS5, Switch, Xbox One, and Xbox Series X/S.