Sai-hee has not been described and has not appeared in person.
Besides the ideals that her organization holds, little is known about Sai-hee's personality. These ideals revolve around the reaping of souls, primarily allowing unaligned Reapers without Servants to reap the souls of all dead, including those killed by Abolish, in peace.
Sai-hee is about 600 years old as a servant, and either rose to power in or founded the Independent faction.
Sai-hee's power is currently unknown, although it is obviously very powerful.