Yasuko's profession is simply being a hostess and an entertainer at a bar named Bishamontengoku. You are correct in that she is not a sex worker. Think of them as modern Geishas. Their main duties are to entertain customers, like singing karaoke with them or cajoling with men. They interact heavily with the customers, unlike the servers or waitress at a normal bar.
In Japan, two types of bars are hostess clubs and kyabakura (キャバクラ?),
a portmanteau of kyabarē (キャバレー?, lit. "cabaret") and kurabu (クラブ?,
lit. "club"). Hostesses who work at kyabakura are known as kyabajō
(キャバ嬢?), literally cabaret girl, and many of them use professional
names called "genji name" (源氏名 genji-na?). Hostesses light cigarettes,
pour drinks, offer flirtatious conversation, and sing karaoke to
entertain customers. Hostesses can be seen as the modern counterpart
of geishas, providing entertainment to the likes of groups of
salarymen after work. A club will often also employ a female
bartender, who is usually well-trained in mixology, and may also be
the manager or mamasan. Hostess clubs are distinguished from strip
clubs in that there is no dancing or nudity.
Hostesses often drink with customers each night, and alcohol problems
are not uncommon. Most bars run on a commission system in which
hostesses receive a percentage of sales. For example, a patron
purchases a $20 drink for the hostess, most of the time which are
non-alcoholic concoctions like orange juice and ginger ale, and the
patron has purchased the hostess's undivided attention for the
subsequent 30-45 minutes. The hostess then splits the proceeds of the
sale with the bar 50/50. The light or no alcohol content of the drinks
purchased allows the maximum profits and assures the hostess does not
become intoxicated after only a short time at work.