Just because you pay your share of the rent on time, doesn't mean you are immune from late fees and possibly eviction when your roommate continues to pay their rent late.
If two people sign a lease agreement they are both responsible for the total rent amount. Even though you may have something written up between the two of you, you will still both be responsible for paying the full amount each and every month. This is a common issue between housemates in share accommodation. Because your housemate does not keep up to their end of the private agreement by not paying by the due date, you are both still held accountable. The Landlord expects the full amount each month and is not concerned who pays it as long as it is paid as the lease agreement states.
Considering you could be evicted due to your roommate's poor financial planning, you should make it a priority to determine if that person would make a good roommate before you move in together. Take the time to determine whether or not they will be financially reliable