I personally find Reg's whole approach creepy.
I quite like him at age thirteen - his gallantry towards Phoebe, his ambitions, his resentment of the CS crowd moving in and taking over. But what we see of him as an adult is very much about the fact that he wants Len, and is very determined to get her before she goes to Oxford and meets other men.
We get a brief mention in Triplets that he's new to the Platz. In the very next book, Joey is gossiping to Grizel about Reg's interest in Len. At this point, Reg has just formally spoken to Jack about wanting to speak to Len in "a year or two time". Len is 16 and a half, and Reg is 26. Joey at that point is pretty sure the marriage will happen eventually. Grizel inspects Reg, and approves of him for Len.
In Redheads, Reg is running the lights at the Christmas pageant.
In Adrienne, Len and Reg share meaningful looks after the fire, and Len "realized that her future was settled, once she had finished her formal education". This is the first time we've seen Len and Reg interact, ever.
In Summer Term, Reg is introduced as "was a close friend of the Maynards. Indeed, he had hopes that one day in the future he might become a relation of theirs. " as he comes to rescue Erica from a hole.
In Althea, Reg gives Len a ride after she face-plants onto her paints, and is introduced as
He liked all the three elder girls but for the past two years Len had been his favourite. Lately he had realised that when she went to Oxford she would miss her very much indeed: a distant friendship would not satisfy him, he wanted something much more than that. He knew, however, that Len had no such thoughts.
This is the second time we've seen them interact.
In Prefects, Reg rescues the prefects from a flood. Later, there is gossip among the seniors about Len and Reg. The triplets have a discussion in which Margot and Con are quite sure that Len and Reg will marry, but Len says "Nothing is settled and I don't know if it's what I
shall finally want."
Len and Reg then have a brief interaction during the Sale, which involves Reg giving meaningful looks and Len going pink, and him musing
"I wish she'd grow up," he said to himself. "It's all very well being matey, but I want more than that. A heck of a lot more than that." However, it was no use wishing. He could only wait and, as far as possible, see to it that no one else took his place, and it wasn't going to be easy once Len had gone to Oxford.
Mary-Lou muses to Vi that "I rather think Reg
will get what he wants. He's certainly got all his wits about him."
Then we have Reg missing, Len freaking out, followed by an engagement, and
Len nodded. "Yes, I found that out when Reg went missing. I don't want to be married yet. I want my college course. A degree is a useful sort of thing to have, particularly in these days. Once I've got that if Reg still wants me then I'm his."
"What do you mean exactly by that?" Jo demanded. Then she added, "Mind you, Len! You're not going on playing fast and loose with that poor boy."
"I don't mean to," Len said. "If nothing else will satisfy him, I'll be engaged. But I won't be married at once."
Len and Reg may have a comfortable relationship of equals in the holidays, but we don't see any of that. What we do see of their relationship is Reg deciding he wants Len when she's old enough to consent, and doing his best to make sure he get her before anyone else gets a chance, or she meets other people. Everyone else who knows about it decides that Len and Reg are a thing that will happen once Len grows up enough. But Len seems to be playing catchup - she's the last person involved who actually makes up her mind, and even at the last, there are caveats, and it's still framed as being all about what Reg wants.
Compare that to Joey and Jack with a similar age difference - we see lots of examples of Joey and Jack having an actual friendship, and interacting on a relatively equal footing. Jack waits until Joey is 20 and has some experience outside of school - a couple of years around the Sonnalpe, some substitute teaching, an extended trip to India - and until *she* spontaneously sees Jack as romantic interest.
And Joey's also a very forceful personality, who would have no problem rejecting someone she wasn't interested in. Len is a conformist who is used to pleasing authority figures. Once they were engaged, if Len went off to Oxford, and realized that the absolute last thing she wanted to do was to marry Reg, move to the Platz and settle down to having lots of babies, I think she would still feel forced to go through with the marriage. I think it's telling that Len says "if Reg still wants me" not "If I still want him."