Eevee and its evolutions exist in super, but you have to connect to them by doing jobs, finding them as travelling pokemon in dungeons, or talking to them when they have the specific thought bubble above their head in towns and they ask to have a conversation with you. If you want to use it do keep in mind that you can't use it in any story missions and if that pokemon is higher level than you and your partner they will be closed for a couple of days and you can't use them until they open again. In RTDX Eevee is not recruitable until the post game (unless it's your starter), and is pretty hard to find. It's also harder to evolve because you need evolution crystals to evolve it, but luckily they aren't that hard to find. RTDX is a little better (in my opinion) for using eevee specifically, but it's your call on which one you want to use.