Well, lets do this.
: Lets start with Princess Peach. Peach has QFR, JCFF, float, veggies, and aerials to approach, bait, mindgame, be defensive & offensive, and edgeguard. Since Rosalina is tall and a huge target for combos, Peach has her down throw, d-tile, aerials, and veggies to work with those. Peach bomber, down smash, f-air, d-airs(maybe), tennis racket, and back-air can knock Luma and get he/she off stage after she split you and Luma up with d-airs, f-airs, back-throw, and forward smashes. Don't overuse the GP on her veggies, because she can bait that and punish you. Don't ever GP her bomb-omb because the bomb will and blow up on her when you pull it in so air dodge to catch it, spot dodge, avoid it, or let Luma take the hit. Peach's jiggling game is decent since Rosalina is floaty and has unsafe landings(especially without Luma) so mix up if you can. Peach got up-airs, veggies to bait air dodges or you grabbing the item for her to punish, up-tile and up-smash(slow, but good for hard reads), and other mix-ups such as grab and aerials. Don't challenge Peach's f-airs and up-airs with your aerials because they're not good against her and can get baited or beaten out. Edgeguarding, this is where Peach can do well at since Rosalina has no hitbox on her recovery. Her veggies can help in the long run and her aerials(especially n-air, f-air, and back air) can gimp Rosalina and give you trouble coming back. Down-airs for stage spike on her recovery, same with back-air. Down-smash, down-airs, and golf-club are something to watch out for when you grab the ledge twice and her ledge trump.
: Now with Rosalina. Your zoning, range, spacing, and with Luma can give Peach problems. Jabs, back-airs, n-airs, dash attacks, and star-bits even if used correctly can shut down Peach's floating and approaches. Because that Peach is slow on the ground while your ground mobility is superior, she can camp(alittle), catch her landings more than Peach can, zone her out, out-space her with Luma, and pivot smash and tile for mix ups. Her ground game is better than Peach's with good frame data and with Luma on your side. Rosalina can kill better, especially with rage and Luma if she lives longer, keep Luma alive, and avoid getting knocked off stage. Still light though, but so as Peach. Rosalina's juggling game is stronger because she has her dangerous up-airs to juggle, combo, and kill Peach early since she's light and floaty. Because that Peach's floaty(alittle more than Rosalina), Rosalina can make it more diffcult for Peach to land safely than she can with up-airs, up-smash, and up-tile, anti-air moves as well and other mix ups. As long as Luma is alive and with you, then Peach won't combo you for free, make it difficult for her to approach, score early kos with Luma because Peach is light, and zone her out with aerials and tiles. Edgeguarding, Rosalina's down air can trade with Peach's up-b if timed right to spike her down. With Luma on your side, she'll have to mix up her recovery with bomber, float, and up-b to avoid your down-air, especially Luma's hitbox because it can kill Peach early with enough perecnt on her and rage. Forward smash with Luma on the ledge can score you a kill if you get a read and back air if Peach recovers low. Down smash and down tile for ledge guarding as well and the ledge trump for Rosalina back-air. Watch how Peach recovers first, get a read, and then make your move because her recovery is better than your and you don't want to lead yourself to get gimped.
Stages: Helberb and Delfino Plaza, pick those because it favors Rosalina for her to juggle and pressure under platforms and she can score early vertical kills because of the low ceiling. Battlefield even, but I think it may favors Rosalina alittle more because while Peach can use the space to stay on Rosalina and use the platforms for extending combos, mixing up recovery on them, and juggle and pressure under them, Rosalina can do the same because of her range and tall height, she got the advantage for juggling, pressuring, giving Peach more trouble landing, and using Luma under platforms for traps, mindgames, and set ups. Dreamland same, but with more space for Rosalina to run around and low ceiling for her up-airs. Plus, the wind as well. Final destination is a good stage for Peach to go in because of no platforms. Lylat Cruise, tiling can mess up spacing and Rosalina's recovery, but good for Peach because of the platforms to pressure and extend combos.
Customs: I say Rosalina got the advantage here on customs. Shooting star bits, Guarding Luma, and teleport Luma can help Rosalina more with zoning, camping, spacing, set ups, and giving her a projectile which can cause problems for Peach. With Shooting star bit, Rosalina not only has a projectile for zoning out Peach with long range, to camp, and force approaches, but Peach will have to be careful with picking up veggies because of it. Guarding Luma for protection from incoming attacks, to mindgame, and to space Peach out. Teleport Luma for baiting landings, mindgames, set-ups, especially under platforms, and for edgeguarding. I think the light veggie and any bomber, the fire one especially will be to go for her, but that's only if she can get through Rosalina's customs from zoning to use them for her combos and set-ups.
Results: The Mu on default is +1 in Rosalina and +2 for customs.
has the combos, the pressure, float, aerials, veggies, and edgeguards to take on Rosalina; However, because of
's stronger juggling game, superior ground game, mobility, ko potential, zoning, spacing, and Luma to shut down Peach's approaches, Peach will have problems landing, getting in, and dealing with your disjointed range. Plus, customs will make it worse beacuse they will help Rosalina more for her zoning, spacing, camping, and other tools which Peach can't handle. If you faced a good Peach player who know her really well and the Mu, then you'll be surprised that she'll give you a run for your money; But for you, if you know how to fight Peach, then you'll do well against her and give her trouble. Good luck, fighting good Peach players.
