The Twins' Mother

The Twins' Mother in an unnamed character mentioned many times in the How to Train Your Dragon (Franchise).

Not Allowing a Tatoo
While the teens are gathered around a fire cooking, Tuff brags how it's his destiny to kill dragons, and reveals a mark somewhere on his body add proof. At seeing it, Fishlegs Ingerman asks in surprise if his mother let him get a tattoo, to which Tuff responds no, that it's a birthmark. Ruff immediately insists that it's fake since they've known each other since birth.

Reference as the Birthgiver
Their mother is mentioned indirectly when the twins are both caught in a net and Tuff states that the situation is "worse than the womb".

Missing the Twins
Stoick orders Hiccup to get the "troublemakers" off Berk, referring to Bing, Bam, and Boom. But Tuff misunderstands and, in thinking the chief is taking about him and Ruff, replies that their mother will miss them.

Broken legs
Tuff uses a phrase he thinks goes like "You can't make am omelette without breaking a few legs". Hiccup corrects him, and Ruff suddenly declares that they have some apologies to make. Tuff suggests they'd better start with their mom before they both run off.

Another Reference as the Birthgiver
When the twins are frozen together in Death Song amber, Tuff again says that the situation is "worse than the womb".

Relationships
The twins' parents together raised them as if they were a single person, as is mentioned in "Twinsanity", and deprived them of much individuality. They also had little sense of sentiment when they gave the twins a pet yak only to feed him to them later and give them coats made from his hide.

Ruffnut
Though Tuff says she's not the best, Ruff does know how to cook, and it may have been their mother who taught her.

Tuffnut
Tuff and his mother seem to have a close bond given how protective Tuff usually is when mentioning her.

The Twins' Father
Ruff mentions in "Snotlout Gets the Axe" that her family isn't the marrying kind, so their parents may not be officially married.