This is an Olivia that I summoned. The nature I got for her is -atk +spd, which isn't that bad since she's not really an offensive unit, and the speed might help her avoid getting doubled. I currently have her at 4* level 21, and she's still annoying to level up. She doesn't hit very hard, and doesn't have the speed to double most units. I hope that changes once she gets higher leveled.
Wow, that sucks and reminds me of both Lon'qu, one has low attack, high resistance and the other high HP, low defense, and the crappy 3* Subaki from 5+ summons ago who had the same nature as your Olivia. Basically, all 3 are not doing enough damage for the amount of damage they're taking. The first Lon'qu had average defense, but he's still dishing out too little damage, the other is taking way too much damage and against green units of all things, and Subaki was at a point where he wasn't doing any damage.
I've never had a 4* Lon'qu, but both Firs I got in my first and current saves have been more stable and useful than him while being the same rarity. For Subaki, I think in my first save he was fine, but in this save, 3* Subaki sucks while 4* Subaki rules -- MVP of the Ursula map for me. It helps that 4* Subaki has high attack in return for low resistance which kind of sucks since he has naturally low resistance compared to the other Pegasus Knights, but his increased attack along with his naturally high defense and speed makes him a really good unit. Subaki's basically the pseudo-lance-wielding Wyvern Rider we could have had if Heath or Vaida were in the game. So, yes, between 3* and 4*, Subaki is much better, but 3* Subaki had low attack while 4* Subaki has high attack, so there's that too. It really depends, though. For example, Alfonse and F!Robin suck as a 2*, but is significantly better as a 3* while units like F!Corrin and Laslow are great as 3* units and can even keep up with 4* units. Add in stat variations and it becomes even messier.
The funny thing is that an average Subaki's attack is actually lower than Olivia's. With high attack, he's only 2 points above Olivia's average 39. But, Subaki has a Sapphire Spear, is much more physically bulkier, and is for all intents and purposes, an offensive unit. Olivia's more a of utility unit. That said, because Olivia's HP and attack are naturally low, speed naturally high, and while her overall defense is average, having lowered attack for increased speed is both harmful (attack) and pointless (speed). If you can still get a special map Olivia or have one, I'd recommend you train that one instead since she'd be average, so she wouldn't have an outstanding strengths or weaknesses other than it's Olivia, it's her design, so she'll have those strengths and weaknesses.
On another note since Subaki was mentioned and he has a Sapphire Lance. Of the color weapons, I think the Emerald Axe is the prettiest. Unfortunately, only Arthur has it unless you got Narcian. Sapphire Lance is all right. Kind of wished they made it a naginata just 'cause. As for the Ruby Sword. Well, I'm just going to leave this here.
Whenever I see the Ruby Sword, I'm reminded of Soul Edge's appearance in Soul Calibur IV. The funny thing is because Hinata has Fury, it kind of functions like Soul Edge or the ultimate version where it drains you health during fights. Oh, poor, idiot Hinata. Welp, I guess that makes the Sapphire Lance Soul Calibur then.