I think Lightning Fire Dragon mode counts as a protagonist buff because it initially started like that when Laxus gave him the lightning
that also fits Hiro´s vison of Natsu getting protagonist boosts
Lightning Fire Dragon a protagonist boost
Fire Dragon King Mode with Igneel flame a protagonist boost
Fire Dragon King Mode ( without Igneel flame ) a protagonist boost
Lightning Dragon Mode a protagonist boost
Dragon Force plus the combination of Atlas Ignia Igneel a protagonist boost
I don't think FDKM should be counted as protagonist buff like the rest. It's a mode that Natsu obtained during the 1 year timeskip while he was trying to master the leftover flame of Igneel, he said it in chapter 464 he trained for 10 months to master Igneel flame and FDKM is a by-product of that training. If we count it as a protagonist buff for Natsu and that without it he would only be on Gray level with his base level than we should also remove devils slayer mode from Gray, Stardress from Lucy, all the enchantment buff that Wendy gave to Erza and red lightning from Laxus as well as whatever power he obtained from eating Elexion soul to make it fair.
Because if people were to count that from those characters as something they achieve through their hardwork and training and yet Natsu isn't allowed to count his FDKM as something he achieved from his training than that is simply hypocritical in my opinion. If that the case then Natsu is forever limited to having growth from his own effort, training and intelligent like for example him using FDKM scales for defensive purpose to block Suzaku attack etc.
I mean Lucy originally got her Stardress from CSK later on she trained to master it and even went further with Stardress mix, Gray originally obtained his devil slaying from Silver later on he trained with it and improved his skill in utilizing it, Laxus accidentally obtained red lightning during his fight with Wahl later on he trained how to utilise it better etc. Same case with Natsu, he obtained LFDM from Laxus then he trained how master it on his own and the flames which Igneel left him with inside his body Natsu later trained for 10 months to master it and along the way he created FDKM to go with it. So how was that not counted as Natsu own power and part of his base level and yet when it comes to other characters they are counted? That is very unfair IMO.
