Is she saying "This... doesn't look attractive?" or something similar?

is there a name for this type of swimsuit or underwear that doesnt cover the anus, but just the pussy?

Translations checked/fixed. Enjoy.

And god, that is one hot swimsuit/whatever.

What is a "banchou"? It was used in the same doujinshi in the Rise story, also to refer to the hero. It has a sort of Yakuza ring to it for some reason.

Also, Lalaca, one thing I'm confused about with her first line: "takai desu". Doesn't "takai" mean "high or expensive" normally? Obviously that makes no sense so can you explain please?

Banchou is one term to refer to a leader of a certain group, so our protagonist in this game. As you mention, it has some minor connotations to yakuza, but the most often used word in that case is kumichou (meaning something like 'clan leader').

Her first line is actually written as 「あったかい」, which is another way to write/say 「あたたかい」. It's a more casual form, in one sense, and has somewhat of an emphasised expression, due to the double consonant sound represented by the small 'tsu'.

I see, thank you. What threw me off was that I thought the っ was being used in this case as an exclamation, so I thought it was "Ah! Takai" rather than "Attakai", as you could probably guess.

Slang is one of the trickier parts of Japanese.

Also, a banchou typically refers to the leader of a delinquent gang, especially the "top dog" of his school/area. In this case, the author's referring to how Kanji is all gangster-ish.

Actually, that's another thing that would have confused me: the author is actually using "banchou" to refer to the main character, not to Kanji. You can see this is the associated Rise page when he labels all the dicks surrounding Rise.

To help alleviate the confusion, banchou is a nickname given to the MC on a lot of futaba boards, as his persona, Izanagi, dresses like the head of a delinquent gang.