Analyze the relationship between 'Syriza' and 'Alexis Tsipras' within the context of the input sentence.
Options: platform, operator, chairperson, legal form

ATHENS (Reuters) - Rebels angered by Greece's international bailout walked out of the leftist Syriza party on Friday, formalising a widely-expected split after leader Alexis Tsipras resigned as prime minister to pave the way for early elections.
chairperson