This article explains the various stages of character evolution.
There are four stages of evolution:
This means that, depending on the character, a knight can undergo one to three increasing evolution processes. The stage progression varies based on the knight's rarity.
|Undergo RG (Blooming)|
|RG (Bloomed)||RG (Bloomed)||RG (Bloomed)|
Characters evolved to the bottom-most row in that table have about the same effectiveness. Hence, a bloomed 6* is about as good as a 2* that underwent rarity growth (blooming).
Individual characters are not necessarily able to undergo all available evolution processes. For example, Serial Code code characters tend to be 5*s that cannot undergo blooming. Likewise, a large fraction of 2-5* knights cannot undergo rarity growth. The devs intend to give all characters the capability to go through all evolution processes, so you just have to wait if there's a girl you want to power up.
Cost and Effort
Although getting stronger is vital to progression in the game, you'll likely want to put some forethought into who to evolve and when.
Evolution is available to all characters. It requires Blums and gold, both of which are very easy to obtain. There's a lot to gain from evolution, so you should evolve knights ASAP. Even a new player should be able to evolve a few characters within the first day or two.
Blooming is available to the majority of 5*s and 6*s. 2-4*s cannot bloom. All new 5*s and 6*s can bloom except Serial Code 5*s. It requires Fururus, petals, blooming medicines, and gold. All of those can be obtained in the normal course of playing the game, but there's a significant amount of work involved in getting these materials. Players will definitely want to bloom any and all characters they intend to use frequently, but anyone attempting to bloom everyone will have to put in an absurd amount of effort.
Rarity Growth is available to most characters except 6*s. RG makes the character become a 6* which is why a 6* cannot undergo RG. For 5*s, the capability to bloom is a prerequisite to being able to RG. RG requires RG stones which can only be obtained by using the game's premium currency, flower stones, on Gacha. As a result, it is effectively paywalled and players wouldn't have the resources to RG anybody and everybody. RG should only be done on characters you love or those you think will be absolutely vital in battle. For ideas and examples, see Team Building.
Rarity Growth (Blooming) is an extra step to RG that is specific to former 2-4*s. When you RG a 2-4* character, they become a 6*, but have the effectiveness of a bloomed 5*. In order to make the former 2-4* character as effective as a real 6*, you have to make the character undergo one last evolution process. The process requires the same materials as blooming a 6*, so the resulting stage following RG is called RG (Bloomed). This process is available for all the RG-able 2-4*s, so there's no reason to skip it!