The tenant is probably only one of multiple people who are named defendants in this lawsuit. You should be one too.
You answer on your own behalf, and the tenant answers on his own behalf. If you haven't been served yet, you probably will soon.
The tenant is typically only added because he has an interest in the outcome of the foreclosure proceeding. That is, he might be homeless after the foreclosure sale.
In response to the question in the title of your post, you are apparently being sued for foreclosure, in which case a lawyer is recommended.
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If you were not served or served by substitute service as provided by Florida statute, you do not need to respond. However, I would suggest that you hire an attorney well versed in foreclousure defense to review your options.
You do have alternatives such as a short sale, deed in lieu of foreclosure or bankruptcy.
It sounds to me like you have a lender seeking to serve you, the borrower, with a foreclosure complaint at the property securing your loan which property you now rent. If this was your normal place of residence at some point and if your tenant that accepted the papers is an adult and was informed by the person delivering the papers what was contained in them; THEN - you may have been legally served and may be subject to the filing deadlines and other requirements as a party to this lawsuit. Gather all the papers that your tenant received and review the specifics of exactly what happened with a lawyer ASAP to determine whether you were legally served or not.
You will likely be served with a summons yourself. The tenant may retain their own counsel but you do not need to act on their behalf. It may be in your best interest to speak to an attorney if you are served and wish to defend the foreclosure.