Hey there
"Sweet Affection" is purely built around love and affection - Yes, there are side characters and main characters, but apart from M&S (main), they are all separated from one another. Because of the separation of their stories, you will be able to meet all the characters you want (or not), without it being a problem for others.
So, no... No Harem Route in this version of the game and under this name.
Why do I say "under this name" you might ask?!
Well, there are plans to deliver an overhaul in the form of DLCs which will differ the game from the main route.
Please note that BA-Games is not fully committed to this idea, it depends on player feedback.
(stated in the linked post below)
Read
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for more information about that topic. (Sections DLC & Overhaul at the bottom)
There are possible routes listed as well.
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As far as the gallery goes - There is a full-blown gallery in which you are able to replay all the events in the game.
There's also a built-in hint system and a cheat system.
No 3rd-party-tools are needed - everything the gamer heart desires is already in there.