1) Attender 2) Auditor 3) Beholder 4) Hearer 5) Listener 6) Looker 7) Noticer 8) Observer 9) Spectator 10) Viewer 11) Visualiser 12) Visualizer 13) Watcher 14) Witness 15) Witnesser 1) Beholder 2) Human 3) Individual 4) Mortal 5) Noticer 6) Observer 7) Person 8) Seer, of a sort 9) Somebody 10) Someone 11) Soul 
• (n.) One who perceives (in any of the senses of the verb).
Per·ceiv'er noun One who perceives (in any of the senses of the verb). Milton. 
[n] - a person who becomes aware (of things or events) through the senses
percipient noun a person who becomes aware (of things or events) through the senses