D'Argo is essentially the brute force behind the Farscape crew. Being an honourable warrior, he has killed before, but does have remorse for those he kills.
When a warrior with the Luxans, D'Argo had done many selfless deeds, including forging tattoos of a general on himself so he could divert the attention of the enemy away from his countrymen. Hence, he can often be mistaken as a Luxan general from the tattoos he still carries, rather than an ordinary soldier which is what he was.
Imprisoned for a crime he didn't commit, D'Argo had to send his son Jo'thee away before he was captured as well. D'Argo's wife Lolaan was Sebacean, and her brother Macton a Peacekeeper. Following the Peacekeeper protocol of abhorring half-breeds and keeping the bloodlines pure, Macton saught to break up the partnership of D'Argo and his sister, and eventually kill their hybrid son J'othee. This lead to Macton killing his own sister and putting the blame on D'Argo, framing him for his wife's murder.
For this reason D'Argo sent J'othee away when he was very young, so he would not be caught and imprisoned, possibly executed for being a half-breed. D'Argo was indeed imprisoned, and spent many cycles under the thumb of the Peacekeepers, simply being tortured, put to work in mines and various other jobs. It was on a Leviathan used at that time merely for prisoner transport, called Moya, that D'Argo managed to escape with fellow prisoners Pa'u Zotoh Zhaan and the deposed Dominar Rygel XVI, John Crichton and the defector Aeryn Sun.
Early after the escape it seemed necessary for D'Argo to assert his position as the leader of the crew, the "alpha male" of the pack. Again, this may have something to do with how he tends to follow his instincts all of the time. At several points though he has fought against Aeryn for leadership of the crew, seeing as though the two are warriors. The same has gone for Crichton early on, who has never really been a warrior, but seems to elect himself leader a lot of the time. However, both of these people have turned more into comrades and friends for him who he does not have any rivalries with.
With the arrival of Chiana on board, D'Argo could see someone who made him look at the universe differently. Whilst he was bound by honour, Chiana had no such obligations. In the end they turned out to fall in love with each other.
When Stark rejoined the crew and brought D'Argo the information of Jo'thee being a slave, D'Argo thought Stark partly responsible. However, when Stark seemed to sacrifice his life for the others (to rematerialise later) and help them rescue Jo'thee, D'Argo became grateful to him. But when Jo'thee had already been purchased by Scorpius, D'Argo was willing to turn Crichton in to regain his son, but managed to keep himself from doing this.
But with J'othee and D'Argo reunited, D'Argo could see exactly how his upbringing - or lack of it - had affected his son. But with him preoccupied with J'othee, D'Argo's relationship with Chiana also took a beating. In the end, the rebellious natures of both Chiana and J'othee brought them together, and D'Argo ended up losing J'othee and losing his relationship with Chiana.
However, it seems that D'Argo has forgiven Crais for not testifying against his imprisonment when he could and should have.
D'Argo remains a warrior though to the end, and a very capable one. He does not try and push his opinions on the crew as much as before, but is quick to take command, even if his decisions are not the correct ones.