When he meets the Russian, Marlow has already had his civility and faith shaken by his experiences in the African jungle. The Russian, who has been traveling in Africa for two years, appears as another civilized man untainted by the horrors that so damaged Kurtz and, to a lesser extent, Marlow. In this manner, he acts as a link to the "normal" world that Marlow has left behind, and Marlow admires the Russian's ability to live and interact in Africa without letting it damage him.
His need was to exist, and to move onwards at the greatest possible risk, and with a maximum of privation. If the absolutely pure, uncalculating, unpractical spirit of adventure had ever ruled a human being, it ruled this bepatched youth. I almost envied him the possession of this modest and clear flame.
(Conrad, Heart of Darkness, eNotes eText)
In the Russian, Marlow sees the man he used to be, and the man he wishes he could become. At this point, Marlow is irreparably damaged by Africa, and he has no way to regain his own innocence, but in the Russian, he can see himself as he could have been years back. The Russian gives Marlow some hope that he himself could recover from Africa, and that even the horrors of the jungle and Kurtz could be limited in their scope.
Marlow immediatley seems to connect with the Russian, as he seems to be a 'figure of reason' in this chaotically eccentric world of Kurtz. They both are sailors, and so there is an innate sense of kinship over proffession. However I think the central basis for Marlow's respect of the Russian, is based on the Russian's respect of Kurtz. The Russian (not one of the typical Pilgrims by birth) shares Kurtz's disregard of "the Company", and though the two never actually talk in the book, you can tell that this Russian has helped him through illness, and symbolises a more subdued version of the colonialists, which if anything, stops Kurtz's madness decending further. So although the Russian's occupation is medically supportive, he is also a symbol of reason that is keeping Kurtz in the void between heroic explorer and "savage".