Leonid was a Russian scientist and also a cosmonaut. He was very popular with other cosmonauts and is mentioned several times in the story and in cortex scan. Leonid is unarmed and wears an orange suit.
Loenid is bodysnatched by Cryptosporidium-138 in order to convince the cosmonauts that the Blisk are trying to take over. If successful, the Blisk barge in and Crypto must escape his body and make a run for it. Leonid is most likely killed in the following chaos.