Stats can be improved through a number of ways for example , abilities, training, treasures etc.
every rank has a power limit, for example the limit of stats at rank F is 100 in every stat, for rank E it's 300 in every stat, for rank D it's 600 in every stat and so on...
After reaching the limit it's extremely hard, close to impossible to improve the stat without ranking up.
(for example Rank F)
Reaching the limit of a stat in the rank is Called Rank limit, (Limit Rank F)
Reach the limit of all the stats is call in a rank is call Rank extreme, (Extreme Rank F)
Exceeding the limit of any stat is called Ascendant rank, (Ascendant Rank F)
Exceeding the limit of all the stats is called
Transcendent rank (Transcendent Rank F)
*it's extremely hard to reach the level limit, but it can be done with extensive training and/or different special resources.
But exceeding that, reaching the ascendent much less the transcendent rank is infinitely harder, close to impossible.