Why is 伴 used for buddy?
伴  is a 形聲字 (Semantic-phonetic compound character) 

The 亻 radical means 人 (person/ people) ; it is the semantic component of the character '伴' ( the word 'companion' is related to 'people') 

半(bàn) is the phonetic component of the character 伴(bàn)


  Is it true that 伴 comes from the word, 半(half) and thus can be interpreted as (significant other person)?


No, you can have many companions, and a servant accompanies you is not your significant other