Many cost 65 and some other high-cost units have another form after their 6★ awakened form called EX.
EX units (EXユニット) were first implemented in January 2021, and a few new ones are added every month. Brand new EX units are released, alongside new EX evolutions that have been retroactively added to existing units.
Example: The cost 65 Wright & Wright (#15013), first released in June 2021, only exists as an EX unit.
Example: The much older cost 65 Helmholtz (#2177) was re-released with an EX evolution in September 2021.
Similar to awakening, if a unit isn't already available as an EX unit from the gacha, you have to evolve (進化適応) them. This takes an incredibly long time, because evolving a unit requires 20 mutation rabbits (ミューテーション・ラビット), and you can only get one of those per day.
However, EX units have considerably higher stats and more abilities than other units. When evolving a unit, their base stats increase like so: HP is doubled, and attack and recovery are both increased by 50% and rounded up. They also have a leader skill, which will be in effect if you have that EX unit in the first or fourth position in your team.