They have several romance lines with different length, heat and character requirements. From their website: “What we’re looking for now: Big, high-concept historical and contemporary romances.” It’s my understanding that they only respond if interested, otherwise there will be no reply or confirmation. This is an ebook imprint, there are other Avon books that require an agent. Note: their other lines for longer novels require an agent. They have contests like So You Think You Can Write that might be another good way in. Their wait time to hear back about a submission can be six months or longer. Don’t let the short length fool you, their requirements are very specific for each line and it’s wildly competitive. They are the short paperbacks found on grocery store racks. The only way into Harlequin without an agent is to submit a “category romance”. If holding a paperback in your hand is part of your dream, make sure to seek out the publishers that will offer that as part of the deal. Some of these publishers offer a print and e-book version of your book as part of the contract but some of these presses are strictly digital only. Here’s my list of 22 Romance Publishers Seeking Unsolicited Manuscripts This will help you avoid some of the scam publishers out there.
This is especially important for small presses that you may not have heard of. The cool thing about romance is that there are many publishers that will consider a manuscript without an agent. Before submitting, I recommend reading about each publisher on the Absolute Write forum.