As Jason was killed prior to the start of the series, not much information about his personality is known. He did bully Jughead at school from time to time. However, according to Cheryl , he was the best brother anyone could've asked for, and was a genuinely nice person who didn't deserve to be tortured and murdered. Jason was very protective of his loved ones, especially Cheryl. When they were younger, Cheryl insisted on having separate birthdays from Jason despite the fact that they were twins, until one year, he randomly recommended that they combine their birthdays into one. It was not until years later that Cheryl discovered the reason for this was that no one would come to her birthday. Jason protected her feelings whenever he thought it was needed.
This is an exerpt from Alice in Wonderland.
"You are old, Father William", the young man said,
And your hair has become very white;
And yet you incessantly stand on your head --
Do you think, at your age, it is right?"
"In my youth", Father William replied to his son,
"I feared it might injure the brain;
But, now that I'm perfectly sure I have none,
Why, I do it again and again."
"You are old", said the youth, "as I mentioned before,
And have grown most uncommonly fat;
Yet you turned a back-somersault in at the door --
Pray, what is the reason for that?"
"In my youth", said the sage, as he shook his grey locks,
I kept all my limbs very supple
By the use of this ointment - one shilling the box -
Allow me to sell you a couple?"
"You are old", said the youth, "and your jaws are too weak
For anything tougher than suet;
Yet you finished the goose, with the bones and the beak -
Pray, how did you manage to do it?"
"In my youth", said his father, "I took to the law,
And argued each case with my wife;
And the muscular strength, which it gave to my jaw,
Has lasted the rest of my life."
"You are old", said the youth, "one would hardly suppose
That your eye was as steady as ever;
Yet you balanced an eel on the end of your nose -
What made you so awfully clever?"
"I have answered three questions, and that is enough",
Said his father, "don't give yourself airs!
Do you think I can listen all day to such stuff?
Be off, or I'll kick you downstairs!"
The marquis gave his hand to the young princess as she alighted, and followed the king who went before; they entered a spacious hall, where they found a splendid collation which the Ogre had prepared for some friends he had that day expected to visit him; but who, hearing that the king with the princess and a great gentleman of the court were within, had not dared to enter. The king was so much charmed with the amiable qualities and noble fortune of the marquis of Carabas, and the young princess too had fallen so violently in love with him, that when the king had partaken of the collation, and drunk a few glasses of wine, he said to the marquis: "It will be you own fault, my lord marquis of Carabas, if you do not soon become my son-in-law." The marquis received the intelligence with a thousand respectful acknowledgments, accepted the honour conferred upon him, and married the princess that very day. The cat became a great lord, and never after ran after rats and mice but for his amusement.