The very first opening moments of the trilogy tell us Doc Brown’s Enneagram number. Look at all those clocks! He’s not just repairing or tinkering or inventing, he’s collecting. The filmmakers may have been winking at us about the theme of time in BttF, but they’ve also given us key information about Doc’s character. As a Caractacus Potts type, Doc could be a Seven or an Eight, but this collection indicates he’s a Two.
When he goes into the future, what does he do? He looks up people he cares about — Marty — to see how they’re doing. When Future Marty is a mess, Doc ropes Current Marty into a time travel event in order to help him. Very Heart Type.
It’s possible this is what makes his love for Clara in the third movie so sweet. He’s been solely focused on science, and as a collector he’s had much to satisfy him. His friendship with Marty is dear to him and has sustained him. The way his heart opens to Clara, though, against his will even, is charming. How else do you make someone not classically handsome the hero of a love story? The filmmakers and Lloyd tapped into something hidden but essential in Doc. This twist — the last film is a romance! — works so well because he’s a Heart Type.