My brother lost his 3DS and his super smash bros game along with it, so he is obviously pretty mad. He has a couple other games as well that he has data for (physical copies, not bought from the E shop) and he is really scared that if he buys a new model the the new 3DS and puts his game in he wont keep his characters. We just need to know if he can still play his old 3DS games on a new, new 3DS without a system transfer.
It depends on the game.
Any game that you have a physical copy of isn't tied to your device. It can be played on any 3DS, regardless. If you bought a game on the E-Shop, it's tied to your Nintendo ID/account.
However, some games store their save data on the actual device (ex: Mario Kart 7), while other games store game data on the cartridge itself (ex: any Pokemon game). A system transfer is required for any save data that was stored on the 3DS SD card in order to restore that saved data.
Yes! My aunt plays Pokémon Moon on her New 3DS XL and it works just fine! Usually, the info of the game isn't put on the SD Card. Pokémon game cartridges have the players' progress on the cartridge. It's like how us 3DS users were able to play DS games.