"There is traces of magic that can be followed... The Keys are on Earth. Find them. " - Chronos' very first words in the series
Before the attack on the Timeless Zone
Not much is know about Chronos' past, but it is strongly hinted that he grew up in the Timeless zone, and that he was around when White Lady became the goddess of time.
Attacking the Timeless Zone
Chronos is seen and heard the first time in the series in Episode 1, where he and his subordinates are breaking into the castle, and are chasing White Lady. He appears very calm, even when he finds out that she has sent the keys away from the Timeless Zone, and to Earth, simply ordering his subordinates to find them.
Getting imprisoned in stone
In Episode 39, it is revealed that White Lady sealed Chronos in stone after she sent Aion to Earth, and it is in this form he spends 3/4 of his time of the series, until he achieves The Legend, and uses it's power to free himself.
After getting defeated
In the final moment in Episode 52, it is shown that Chronos got reborn as a young boy, together with Jera, Sibylla and Gero, and he seems a lot happier. He is holding Jera's hand, and seems to me a much happier person.
As a statue
As a statue, he looks much like his normal form, except only seems like he's made up from stone.
After he has broken out of his prison of being a statue, he is shown to have black fur that glistens like stars. He would be described as very beautiful if it was not for his eyes, which are blood red, and the fire of a hell burning behind them.
After getting reborn
After Chronos is defeated and is reborn, he is shown as a 6-year-old boy with red, happy eyes, and pitch black hair.
Chronos is shown to be very ambitious, and he knows what he want. He is patient to a degree, but when it runs out, he gets a really fiery temper. Chronos does also have an aura of power and authority around him, which in turn, makes him used to being heard when speaking, and the only thing he really can't take is being ignored.
White Lady - It is strongly hinted that she and Chronos have a shared past. It is hinted that they were really good friends before she became a goddess, and was also there at the time when she became one.
Chronos (クロノス?) - Chronos is the personification of time in the greek mythology.