Conrad also makes Dain Waris a character in his own right, one who has a leadership position in Patusan and a strong stake in the island's politics. Plus he's Jim's best friend, which doesn't hurt in the power department.
The weird thing is, we don't get many scenes between the two of them. We just have to take Marlow's word for it when he tells us they're buds. In a way, the fact that Dain Waris, an islander, and Jim, an Englishman, are friends in the first place is what really matters in the novel.
In fact, when it comes to race, Dain Waris's character gives us a glimpse into how Marlow (and possibly Conrad?) thinks:
This was true; he had that sort of courage – the courage in the open, I may say – but he had also a European mind. You meet them sometimes like that, and are surprised to discover unexpectedly a familiar turn of thought, an unobscured vision, a tenacity of purpose, a touch of altruism. [...] Dain Waris had a proud carriage, a polished, easy bearing, a temperament like a clear flame. (26.4)
Marlow is certainly paying Dain Waris all kinds of compliments here. But notice that all the positive characteristics he's attributing to Dain Waris are associated with being European. Those good traits are "unexpected" in an islander. This rather disturbing description, with its casual racism, gives us a lot of insight into the era in which Conrad was writing, and again it makes us question whether Conrad is using Marlow as a mouthpiece, or just a window into the times.
Finally, it's Dain Waris's death that brings about the death of Jim. A distraught daddy Doramin can't handle the thought of losing his son, and must have revenge on the man he thinks responsible. It's Dain Waris's heroism in battle that leads to Jim's own tragic death. The two besties die within hours of each other.