... Nele. Nele is Tijl's (girl)friend and was raised by the same mother (although they are not related biologically). Together with Lamme Goedzak they are - apart maybe from Reynard the Fox - the most well known characters in our Flemish fictional history, despite they are not Flemish born.
Tijl Uilenspiegel (with Uilenspiegel being his last name) was born under the name Till Eulenspiegel (which means, just like Uilenspiegel, "Owl's mirror") in a German chapbook in 1515 as a native of Brunswick.
There have however been some attempts to find evidence for the historicity of the person: in a contemporary legal register of the city of Brunswick historians found one Till van Cletlinge (that "van" makes clear he was of Flemish descent, just like the "van" in Ludwig van Beethoven), who was incarcerated there in the year 1339, along with four of his accomplices, for highway robbery (Eulenspiegel is, in the story too, a naughty boy, but a good naughty boy, something like Robin Hood). And in Mölln, the city where Eulenspiegel reportedly died and was buried in the plague year of 1350 a memorial stone was commissioned by the town concil in 1544 which has an inscription in "low German" (which is very close to the Dutch of that time and still close to my dialect these days):
"Anno 1350 is dusse / steen upgehaven und / Tyle ulen spegel lenet / hier under begraven. / Marcket wol und / dencket dran. wat / ick gwest si up eren. / All de hir vor aver / gan. moten mi / glick weren."
("This stone was carved in 1350, and Tyl Ulenspegel lies buried underneath. Note well and remember what I have been on Earth. All those who pass here will become as I am now.")
In that early German version however, our hero did not have a girlfriend yet. Nele was entered in the story by Charles De Coster in 1867, together with his friend Lamme Goedzak (Tame Good Guy). De Coster was born in Munich from a German father and a Walloon mother, moved with his parents to Brussels, and wrote in 16th century French because ... he judged that Flemish manners and speech could not be rendered faithfully in modern French.
His Legend of Thyl Ulenspiegel and Lamme Goedzak was barely read in belgium, because it did, dixit Wikipedia, "not meet up to the conventional standard of Belgian nationalism" (belgium was conceived only in 1830 as a breakaway from The Netherlands, the French speaking people in power had no intention to fortify the identity of the Dutch speaking majority not in power, while belgium was supposed to become entirely French speaking), but became very popular over the rest of the world. And thus Tijl Uilenspiegel (and Nele, and Lamme Goedzak) did become a symbol for Flemish independence anyway ...