She maybe an immortal sun goddess but she is hardly what I would call interesting.
I love celestia! She is so ambigously defined and yet so touching. Her gentle, manipulating, trolling ways seem to be ambigous and goodhearted, yet can confound the minds of those who are determined to sort out a headcanon.
She's elegant, graceful, smart and nurturing in a very light, trusting manner. I think she sees the Main 6 as either her replacements with regards to fighting evil forces or just her eventual replacements, period. They certainly seem to do a better job than she ever did, and i think she realizes the power of their friendship makes the Elements of Harmony stronger with them than it was with Celestia alone (or Celestia & Luna).
She also develops from having to cover for Twilight in early episodes, like "Swarm of the Century" as Twilight is maturing in her new role, to having Twilight essentially cover for her missteps or lack of action in later episodes (e.g "A Canterlot Wedding").
And she seems to take the same everyday attitude to raising Cadence that she does to ruling the kingdom and did to nurturing Twilight, encouraging interaction with the common folk and a levelheaded attitude.
Her only problem is that her power, judgment and ability to use the EoH seem to be waning either with age or simply as the cycle puts the future in the hands of the Main 6.