A dark secret they keep from others (perhaps even from themselves).
Going to answer this from an OOC perspective because neither Oliver nor Avigail would ever open up on a topic like this. At least not honestly.
Oliver’s a bit of a tougher question to answer. For the most part, I feel as though most who have roleplayed with Oliver consider him a silly, sarcastic Miqo’te who, despite being a bit rude, tends to have good in his mind; that his intentions are good. When it comes to people who have roleplayed with Avigail, from what I can tell is that the general consensus is that Avigail is the one who you need to keep a close eye on.
This isn’t true.
Since adolescence, Oliver has toyed with death. When he spent his time as a bounty hunter, he was exceptionally brutal towards his targets, showing little regard for their lives, or his own for that matter. He even toyed with them, experimenting with different ways to hunt and kill.
Of course, for the most part, Oliver’s grown out of these habits. He shows genuine care to those within his company, as well as his wife and his own family (even though he may have pissed her off yesterday). Yet regardless of seemingly changing, Oliver’s mental health could, at any time, plummet.
He has the potential to devolve into a sociopath, and he knows it.
Quite thankfully, Avigail’s is relatively easy to explain: she’s more or less the opposite of Oliver in those regards. She likes to act as though she’s tough, but she’s actually a giant softy somewhere within.
While it isn’t the worst or most horrifying of secrets, Avigail does her damndest to hide the fact that underneath her rough exterior, she’s actually somewhat sweet. It would go against everything she’s built up over time for people to begin recognizing that, so she tends to just sort of drift off alone, repairing the company airship - the Jupiter Jazz - or fiddling with her own equipment.