"One Yuri" means
"a certain Yuri" or
"a certain person named Yuri" or
In other words one is being used like a or an and is used to make an indefinite reference to the person. We might make an indefinite reference because we have never met the person before, and we basically know nothing else about him except his name. An example of this is if a guy named Yuri calls your house and asks to speak to, say, your brother. If you do not know who this Yuri is, and perhaps you also do not think your brother knows, you could say to your brother,
There is one Yuri on the phone for you.
This is very much like saying a Yuri as far as being an indefinite reference.
Even though it is an indefinite reference it is a specific reference. We are referring to this specific person. The specific person on the phone, in this example; or the specific Yuri found slumped in his seat, in the case of the article you cite.
In certain cases, an author will make an indefinite reference to someone because the author feels a more detailed identification is not necessary or desirable for the narrative at hand. For example, the reporter of this story may feel that to identify this Yuri in more detail, such as providing his surname, could put this "one Yuri" at risk of danger, given the volatile context.
There could be other times when a more exact identification of this "one Yuri" is not important to the story, even when the person Yuri himself is important to the story.