Geek & Sundry, of LARPs fame, have created up a few more Firefly character sheets for the DND 5e system. I've had a look at both the original series (released 26 April 2016), and the new set and have my heart set on playing on River Tam. Here's her bio:

River Tam is the prodigy child who makes her brother look like an idiot. Given her high intelligence and skill in the martial arts, we’ve made River a Monk. We’re only using the standard spread here, so we haven’t quite been able to show all of River’s strengths (she really is a highly talented and skilled individual), but this sheet gives an idea of how to play her. We consider River a kind of forced Acolyte, since she did want to go off and learn, but was ultimately tortured as the Alliance attempted to turn her into a psychic assassin. Although monks depend on Wisdom to grant them their Ki, River’s mind is often not lucid enough to do the wise thing. Sometimes she is able to focus, but given her history, we haven’t made her extremely wise here.

Relevant links:

1. Firefly DnD 5e character sheets #1
2. Firefly DnD 5e character sheets #2