Based on the in-game app that all of the Phantom Thieves use to plan their infiltrations, the Persona 5 IM App does everything you'd expect from an SMS app, such as viewing, sending, receiving and deleting messages, with notifications to alert you when a new message is received. You can even change settings to turn some features on or off, or change the colour scheme from the classic red to something a little different!
Disclaimer: installing this app does not make you a Phantom Thief, will not give you special powers and will not force you to go to bed early every night.
Minor update to add a few bits of functionality and fix a few of the most frequently reported outstanding bugs:
A sound will now play and a light will now display when you receive a notification
Text will now automatically capitalise when typing into a text box
Added an option to hide the notification bar
Fixed bug where the app crashed for some people when trying to send a message (this requires an additional permission)
Fixed various other bugs