Encounters is a free tool (for one device), and it will sync across multiple devices and work offline for one dollar a month.
The tool lets busy DMs store NPCs in advance of the gaming session and then run encounters with them using various sets of rules.
Currently supported are 5e (D&D), Call of Cthulhu, Savage Worlds, Open Legend, Stars Without Number and WOIN the system the new Judge Dredd RPG uses.
Dungeon World and Dungeon Crawl Classics are confirmed as future developments, and a ‘generic’ system will let you use whatever ruleset you want, provided you can configure the data to your liking.
Along with NPC stats, RPG Encounters from Heromuster has tabs for Players (handy for initiative order) and encounters.
There’s a sub-reddit at /r/heromuster/ for feature requests and news.