What are the impressions on the characters ?

I'm not exactly sure what you mean here. You wrote "of" the characters so I assume you mean how they felt. Upon reaching the African Congo, Marlow is awestruck. The place is an alien land to him where he observes the indigenous people "mostly black and naked, moving about like ants." The other "white" people he meets seem desensitized to both the strange place and the intense misery. By self-proclaimed virtue of their white skin they see themselves as superior beings exploring the muck of an inferior place full of inferior savages.

Marlowe: Initially the impression of Marlowe is of a man whom has seen a great deal and come to terms with that. He is twice compared to a meditating Buddha, but his physical description of having "sunken cheeks" and a "yellow" and "ascetic" complexion hint that he my have paid a high price for whatever knowledge he may have gained. Once the story has begun Marlowe is somewhat naive and eager for adventure.

Accountant is portrayed as a bit of a gossip, revealing some of the Company's unsavory business practices and showing a complete lack of regard for human life as he complains about the groans of a dying man.

The Manager is duplicitous, as he speaks well of Kurtz, but plots for his downfall.