Karl C. Agathon (callsign "Helo") is a fictional character on the re-imagined Battlestar Galactica TV series, portrayed by Tahmoh Penikett.

[1] The (unofficial) backstory Tahmoh Penikett and Katee Sackhoff came up with to explain a comment from Thrace in "Scattered" that she and Helo "go way back" is the two were old friends for a long time who most likely attended the academy together as cadets and as "drinking buddies".

[8] Valerii's true motive in accompanying Agathon is the participation in a Cylon experiment designed to attempt to create a Cylon–human hybrid offspring.

The Cylons track Agathon's progress toward a military base in Delphi, placing him in situations designed to result in affection and ultimately love towards Valerii.

It is there he is reunited with fellow Galactica pilot Kara "Starbuck" Thrace, who is on a special mission from President Laura Roslin to recover the Arrow of Apollo.

His homecoming is bittersweet; Helo's former friends turn their backs on him due to his love for a Cylon, let alone one whose doppelgänger attempted to murder Commander William Adama.

Helo finds support from Starbuck and Chief Petty Officer Galen Tyrol, who was in love with the original Galactica crew member version of Sharon before her betrayal of the fleet and death.

Admiral Cain's summary trial and death sentence of Agathon and Tyrol bring Galactica to a confrontation with Pegasus.

Karl Agathon marries his version of Sharon Valerii, who is commissioned as a lieutenant in the Colonial Fleet and takes the call sign "Athena".

During an attempt by Galactica to spread a virus to wipe out the Cylon race, Agathon disobeys orders and sabotages a key part of the plan because he considers such an action to be genocidal.

This equilibrium was disrupted when Boomer, the Raptor pilot he had formerly served with, impersonated his wife and allowed him to seduce her (while Athena, bound in a closet, watched in helpless fury) and then abducted Hera for study.

After the abduction, with Athena in a listless depression, Helo begged Adama to let him take out a Raptor for a search-and-rescue mission, despite the risk he would not find her or return.

Hera fled the scene and despite knowing he would die of blood loss unless Sharon attended him, Helo demanded she protects their daughter.

While Karl and Sharon Agathon passed namelessly into history, Hera was eventually identified by Earth scientists as "Mitochondrial Eve".

After stranding him on the post-nuclear attack Caprica, the writers had no further plans for the character and intended his fate to be that he died (off-screen) in the radiation that followed the nuking of the planet.

Helo's full name, Karl C. Agathon, wasn't revealed until the release of "33" — after the writers revised their original plans and expanded the character's storyline.

Then he called to his sons In a harsh voice--Helenus and Paris, Agathon, Pammon, Antiphnus, Polities, Deiphobus, Hippothous, and noble Dius--"