Hi maryyang and welcome to the forum.
You might like to read this other thread which deals with substantially the same kind of claim: viewtopic.php?t=3102
Since you are based in Canada you would be better off getting full legal advice from a lawyer there, as they will have a better understanding of US law. My instinct is that these claims don't actually proceed to trial and work by scaring people into paying a large and entirely disproportionate amount of money to settle the claim. Howver I don't have access to the US Pacer court reporting sysytem which would allow me to confirm this theory. A Canadian attormey might well be able to do that.
However at first sight, I cannot see any reason that this is a valid claim. The words Grumpy Cat have been registered with the US Patent and Trademark Office by a company named Grumpy Cat Limited of Ohio, USA. However none of the categories of goods and services for which the words are registered come close to your painiting. This means that this dubious legal firm are just trawling the internet for instances of the words Grumpy Cat and not looking at the details of the goods being sold. However, this operation is not based on careful legal examination of the facts; it is just about extracting money from unsuspecting and worried online marketplace/shop owners, using the threat of litigation.
If you do not want to contact a Canadian lawyer about this you should submit your Answer to the Illinois court. There is no specific format for this. First of all read this document produced by the court: https://www.ilnd.uscourts.gov/_assets/_ ... nglish.pdf. There are two specific grounds for asking for the case against you to be dismissed. The first is that the Illinois court does not have jurisdiction as you are a Canadian citizen, and the case at issue fails to pass the test set by the US Supreme Court in Steele v. Bulova Watch Co., Inc., 344 U.S. 280 (1952), namely you are a Canadian citiizen living outside the USA and all the activities concerning your Redbubble shop were aimed at Canadian citizens and this acitivity, namely the selling of a painting, caused no detriment to any US interests. Secondly you deny that your use of the words Grumpy Cat amounts to trade mark infringement: a. because they were being used descriptively and were not likely to cause confusion, or to cause mistake, or to deceive, per section 1115 (1)(a) of the Lanham Act as the painting clearly did not depict the registrant's goods, and b. the registered mark is not registered for goods such as paintings or artwork in general which the registrant does not sell or deal in. Futhermore the summons lacks all particulars about the alleged infringement complained of and was issued in bad faith.
Summons from Illinois for copyright infringement
Re: Summons from Illinois for copyright infringement
Advice or comment provided here is not and does not purport to be legal advice as defined by s.12 of Legal Services Act 2007