This app was mentioned in 3 comments, with an average of 2.33 upvotes
No companion app, but I use this app for checking my rank. https://play.google.com/store/apps/details?id=com.rocketleaguetracker.app&hl=en
EDIT: Sorry I assume Android, Here iPhone version: https://itunes.apple.com/us/app/trn-stats-for-rocket-league/id1111330177?mt=8
Rocket League TRN? Its not great in respect to realtime but it works
Pretty sure there's one on Android, but it's really shit. The mobile browser is actually more feature packed.