ぼく, as in ‘I’. Young kids often use this, as do older adults (sometimes after おれ) or when being a little less ‘rough’ than when using おれ.
What I never knew was that accent can play a role with this word. I’ve been told that ぼくwith a rising intonation is how an adult would say it, while a child would use a more flat accent. How come nobody ever told me that before?