Harlan was one of the original four members of the the Brotherhood. Little is known about his tenure as Guardian. What is known is that through deep meditations, he came to develop a deeper understanding of the Chaos Force and how to control Chaos energy. (CSE)
Harlan aided his son Rembrandt in the training of his grandson Aaron, providing him with some knowledge of circuitry and wiring, which Aaron used to prematurely detonate a missile headed for the Floating Island, saving it at the cost of his life. Upon learning of his grandson's death, he mourned deeply along with the other Brotherhood members. Harlan, along with Moonwatcher, felt that the new manner of training Jordan to become a Guardian was wrong and that they should all participate in his training, but Steppenwolf stepped in and pointed it out was better Jordan learned gradually before they increased his responsibilities. Harlan and Moonwatcher acquiesced. (KtE: #25)
Like his fellow deceased Guardians, Harlan appeared before Knuckles the Echidna after he was killed in a confrontation with Mammoth Mogul. There, Harlan explained the afterlife and the Chaos Force to the deceased Knuckles. He was also the one who realized the Chao were coming to get Knuckles instead of Steppenwolf. (StH: #121)
While Moonwatcher was more upbeat and outgoing, Harlan seemed more relaxed and peaceful. When the Aurora Chao came for one of the guardians, while his father was hastily assumed they came for his father, Harlan was the one who realized they came for Knuckles and not his grandfather, Steppenwolf. (StH: #121)
- Harlan seems to be the first Guardian to feature a unique marking on his forehead. Jordan and Mathias have a similar marking on their foreheads, and Sojourner appears to have some kind of metal plate where the marking would be.
- Former writer Ken Penders revealed on his Twitter account that Harlan's namesake was inspired by speculative fiction writer Harlan Ellison. 
A red Echidna, Harlan wore a grey tunic, white gloves and black boots. In the afterlife, he wore a green tunic and green boots. Harlan featured a unique marking on his forehead, a trait he shared with his grandson Jordan and great-grandson Mathias.