If you were not legally married to that significant other on or before Dec 31 2017, if they meet the requirements of qualifying relative dependent, then you can claim them. See https://apps.irs.gov/app/vita/content/globalmedia/table_2_dependency_exemption_relative_4012.pdf for the qualification requirements.
Homeownershio does not determine whether you can file as Head of Household. To file as HOH you need to be supporting a blood-related dependent.
You have not mentioned if your significant other worked or had income or if you lived together all year in 2017. If the SO had income over $4050 in 2017 you cannot claim him/her as a dependent, or if the SO did not live with you the entire year he cannot be claimed.
Am I Head of Household?
https://ttlc.intuit.com/questions/1894553-do-i-qualify-for-head-of-household https://ttlc.intuit.com/questions/2900097-what-is-a-qualifying-person-for-head-of-household
IRS interview to help determine who can be claimed:
https://www.irs.gov/help/ita/who-can-i-claim-as-a-dependent
https://ttlc.intuit.com/questions/3113432-who-can-i-claim-as-my-dependent