I'm fascinated by the concept of Leon and Helena being complementary opposites. While Helena is described as an emotional, impulsive, and volatile person, Leon is described as a calm, rational, and serious person.
One complements what the other lacks.
They are completely opposite people, but with a relatively similar past, and this led to an instant connection and exceptional synergy between them.
I wouldn't be surprised at all if one of Hunnigan's intentions in recruiting Helena to the DSO was for her to complete the emotional part of Leon that he abandoned in the past, while he would complement the rational part of her that she needed to mature.
It's a shame to know we'll never see that dynamic again, because it would be fucking great!