Yoko calls herself a university student and she is very knowledgeable about computers. Outwardly she appears to have a quiet and reserved personality, but the strength of her inquisitive mind emerges with a toughness that surprises others. Once she gets hung up on something, she's a bit slow on the uptake with other things.
It is implied however that she uses this personality as a facade, quietly avoiding other people's problems. Monica is quick to point this out in Below Freezing Point as she holds Yoko at gunpoint. Yoko claims she doesn't know what she's talking about and her testimony is backed up by Greg in Decisions, Decisions, who is aware of her altered memory. Yoko finally admits it to herself in her ending to Desperate Times; that was all she could really do after her, the other survivors, and Rita leave Marvin Branagh behind. While working with the other survivors to escape Raccoon City she is definitely aloof about any trouble her allies get into. She will often refuse to help if one of her partners is bleeding, requires a healing item/ammunition or crawling on the ground. In many of her endings she's also very pessimistic about escaping the city, often saying that she's thankful to be alive (Hellfire, Desperate Times) but that she would rather not put up with the situation any longer.
Yoko seems to have a sisterly relationship with Alyssa Ashcroft. They even share a bad ending in the Decisions, Decisions scenario, where Yoko and Alyssa pair up and expose Umbrella before the city is annihilated by the missiles, killing both. A similar ending occurs after End of the Road, where Yoko and Linda testify against Umbrella in court. It is implied by later games in the series that the latter was what began Umbrella's steady decline following the outbreak.
Yoko gets along well with Cindy Lennox and Kevin Ryman. Mark Wilkins also gets along with her okay. One of Yoko's special items is a Manly Hankerchief, which was apparently from Kevin and includes her comment that he may not look it, but he's actually a nice guy.
Yoko does not like Jim Chapman. Possibly because she tends to be very quiet and Jim is the exact opposite (always talking and screaming). As a result, she probably finds him very annoying. George Hamilton and Yoko don't get along either, though the reason is unknown.
Yoko is more inclined to co-operate than any of the other survivors. She sticks especially close to Kevin and David King, but will follow everyone, except for Jim. She will also refuse to help if he is in trouble. Yoko is the only character who Alyssa will follow reliably. David will often follow too, as will Jim. It is assumed that the latter isn't aware of Yoko's attitude toward him, although he is her worst partner.