Semenov is another scientist in the mobile lab and the assistant of Professor Sakharov, although game-wise, he only makes an appearance if Professor Kruglov is killed.
According to professor Sakharov, Semenov used to be his student back in college in his class dealing with genome experiments.
Shadow of ChernobylEdit
Assistant Semenov is the replacement character for Professor Kruglov, in the event that Kruglov is killed in the Wild Territory. Like Kruglov, he will accompany the Marked One in taking measurements around Yantar. However, he is only fitted with a typical SSP-99 Ecologist Suit, affording him with less protection than Kruglov (who wears a combat made SSP-99M).
Background information Edit
Shadow of Chernobyl 'bio' file Edit
A short bio, that was omitted from final version of S.T.A.L.K.E.R.: Shadow of Chernobyl game describes him as follows: 
- ↑ S.T.A.L.K.E.R.: Shadow of Chernobyl 188.8.131.52, stable_bio_name.xml; id=yan_semenov_bio