I did some math and have useful info on Leons Healing! Read details below

So when I finally got my Bright to 5* I noticed that a team effect that he has is healing the team members by 1.75% of the each members max HP. This also takes into account the HP increase healers give. in Leon's describtion of his 5th skill it mentions he heals Frontline every 3 seconds however when you put him on a team in Fight Club, Mirage cave, or general stages the only Team effect you can see is Public Health which is 10% HP increase as all healers do. When play with Leon in your party you do see your Frontline healed though which means his skill works outside of Blitz but the game does not tell you the % by which he heals Frontline like it does Bright. I'm happy to announce I've figured out the % in which Leon heals. He heals Frontline units by 2.5% every 3 seconds and if the front line shares the same typing as Leon then he heals by 2.75%. so for example my Juno has a natural HP stat of 2940. If Leon is the only healer in party then Juno gets a 10% HP increase. So 2940+294 = 3234. Leon's heal will apply after any HP buffs has been applied. So 2.5% of 3234 = 80. Now let's say for instance Juno and Leon had same type then Leon will heal 2.75%. 2.75% of 3234 rounds to 89 HP. The same type bonus applies to Bright as well and Bright also does his healing on your hunters HP after Hp increase is applied. Brights same type heal is either 1.92% or 1.93%. the math on bright isn't as clean cut as Leon idk how visual shower would round brights same type heal but it IS either of those two numbers for sure.

Note: I have not used Leon in Blitz so I don't know if the % he heals by in blitz is higher. But for the purpose of Fight Club and general stages the info provided above is accurate.

SHORT EXPLANATION: Leon passively heals Frontline by 2.5% of their max HP every 3 seconds with lvl 50 5th skill. Healing is applied after Public Health buff. So it always heals scaled to the highest HP stat of Frontline.