Why is Russia translated to 俄罗斯(E Luo Si)?
the "俄" part seems to have no corresponding phoneme, either in Russian or any of the other language I have looked up.


Yes it's peculiar in Chinese. It's related to the Mongolian.

Reference: 郭文深.俄罗斯国家名称变迁考——从“罗刹”到“俄罗斯”

Summarize it in short:

From Yuan Dynasty, the Mongolian translated 罗斯 as oros (but not ros) followed by the Mongolian pronunciation habit, and then translated into Chinese character as 斡罗斯. In Mongolian, there's no words started from [r], such words are usually borrowed words. Then for these words, in order to make it conform to Mongolian pronunciation rules and incarnate Mongolian vowel harmony, the strong vowel of the first syllable will be brought forward. Then 罗斯"ros" became oros, then 斡罗斯.

In Qing dynasty, the Manchu didn't have direct contact with the Russian, they used the translation (especially the transliteration) of the Mongolian which is the intermediary between them. For Chinese characters they used 鄂罗斯 and 俄罗斯, until the Emperor QianLong unified them into 俄罗斯.