Sympathy

Miyuki is a fascinating character. She's apparently unable to form
a relationship with a man on her own (page 17, panel 5), but she's
strongly drawn to men who become involved in relationships with
women she loves. As I see it, she's a lusty bisexual who conceals
her lesbian kink from everyone, including the sister she desires.

"I'm a hopelessly perverted woman [and] I'm afraid of anyone
I love discovering what an awful woman I am."
(page 13, panel 2)
(page 17, panel 6)

Miyuki clearly enjoys sex with men, but by having sex exclusively
with men who are fucking women she loves, she is also enjoying
vicarious sex with those women. Whenever she sucks Ginjirou's
cock (which she really seems to enjoy), she probably imagines
that she's tasting her sister's pussy.

On the other hand, Miyuki might simply be a lazy, unscrupulous
sexual predator who lets her sister and female friends do all the
work when it comes to hunting for men.

In that sense, she's a parasite, in that she draws all her sexual
sustenance from men who belong to other women in her social
circle.

In any case, I also see Miyuki as a rather lonely, love-starved woman
who wishes she could have a man of her own, as we see on page 20,
panels 1-2, when Ginjirou casually (almost callously) brushes off her
offer to be his girlfriend. That's a truly poignant moment, with a hint
of Deep Shit.

As I said, she's a fascinating character,
and there's plenty of fuel for speculation.

You have a lot of jams. Which is good.