Except where mentioned, all of the below is taken from the book 'the world'The World of iceIce and fire'Fire', so is what the in-world official view is, but this would have changed and things may have been covered up through the ages and the information is sometimes wrong (e.g. it states that giants no longer exist):
- heHe raised the wallWall of the north with the help of the childrenChildren of the forestForest
- heHe is written about in maesterMaester Childer's Winter's Kings, or the Legends and Lineages of the Starks of Winterfell
- heHe 'learned to comprehend the speech of the children'Children', which 'is a tale in itself'
- He is a legend from the ageAge of heroesHeroes
- heHe may also have built Winterfell and Storm's End
- May have been a king or multiple kings of house starkHouse Stark
- He is (possibly) a descendant of Garth Greenhand and Brandon of the Bloody Blade
- heHe (or his son) Maymay have designed the fifth tower, which was 200 feet tall and made Hightower a great house
- asAs a boy, he helped build a castle that defied the gales and storms of both the sea God and the goddess of the wind
- From A Storm of Swords, he may have given the Nights Watch Brandon's Gift
From a storm of Swords, he may have given the Nights watch Brandon's gift
If he wasn't the founder of the Starks, he was certainly an important part. That's if he even exists, as mentioned in the comments of the question, which is also why no-one knows more - he's just so old. He may actually have predated written text, but I don't have a reference for that bit.
