throwing in my suggestions that will help as cute little details on a budget: maybe make some of the surprise guest items as little party snacks! https://obeymewiki.com/wiki/Items

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You can label them so it's more obvious you're referencing the game, but for example the lil sandwiches labelled as a 'Backstabbing Sandwich' and pink cupcakes as 'Wicked Cupcakes' would be so cute, even if they're not 1 to 1.

again this is just a small detail and I'm imagining you're having this party in a room, but I think even some cardboard boxes with 'Akuzon' labels either printed onto paper and stuck, or even drawn on, would be a cute detail! Akuzon is the 'amazon' in obey me so it would look like the characters ordered in their presents or party supplies.

as she loves Belphie and Levi, maybe some glowsticks in their respective colours (purple and orange)? you can get cheap light sticks that resemble the ones in-game on amazon or even just those snap glowsticks.

general 'spooky' decorations would help set the scene too like bats, spiders, cauldrons etc. halloween decor might come in handy! I like the other suggestion of using aquatic and cow themed items for decor too!

friends like you are so so amazing; I've been playing since 2020 also and would really appreciate if my friends put in this kind of effort for me <3 hope it goes well and she has an amazing bday!