What are those red flashes?

[Púlsar]

They appear ocassionally with a sound; sometimes I haven't entered battles after resting and that happens.

[Ichidoge]

It means Brad or certain characters in your party are suffering the effects of withdrawal. In this state, the affected characters' stats will be severely crippled, and usually only their special attacks will do any damage. You can avoid this by giving them Joy; they will also get an incredible boost to their stats and will always insta-crit. However, you won't get the Joyless ending if you keep taking drugs.