Kizaru seems to be 0 fazed by "demon" saturn, including demonic circles (The VAs were tho)
Akainu was talking with the gorosei in the Gorosei room
HKs, obviously, do know about Imu too
So what is confirmed so far in the manga:
VA = aware of Gorosei existence as Fighters
Admiral, Fleet admiral = aware of Goroseis Demonic abilities
Gorosei, HKs, = aware of Imus existence
However, given Vegapunks speech, doberman was questioning the contents of it to Saturn. If he knew imu exists I don't think he would do that, so VAs we can cross off.
In contrast, we get this panel:
This reads to me Akainu is AWARE of already what vegapunk is gonna tell. Akainu seem to be not suprised the slightest but angry and cursing vegapunk.
This means Akainu, as the Fleet Admiral:
A) Was told about it
B) Has figured it out
to me, but speculation