The below is a significant modification of Sara, the Delicate Mortician.-
She wields a long wicked hook and holds macabre blades on her back. Her clothes are stained with old blood and formaldehyde.
The W applies a .2-.26 AP ratio to every other ability once it's skilled.
Note that during Mortify (R1), Chronic Pain (Q2) will only apply the damage of one Exacerbation AoE - even though it damages multiple targets. The total AoE is increased (it stems from every target hit), but only one AoE will affect any one target when considering damage. Note that it does apply Exacerbation stacks to all targets hit.
W active (or WA) will reset the shield if cast again before it fully expires. For a similar mechanic, reference Lux's Prismatic Barrier.
Most of her abilities are deceptively weaker than they look. A character that gains attack speed, a shield, a slow, and on-hit damage from a single ability seems strong until you look at the numbers - other characters often experience quadratically increased power as a player's skill level increases. See Lux's shield - a .35 AP ratio shield isn't that significant; but with a refresh, a .7 ratio shield on 4 members of the team is huge. Sara's abilities seem to do a lot, but the numbers don't do most things quite as well as someone else's.
Note that her abilities gain about 50-55 damage per rank in an optimal scenario. This is comparable to that of most other mages such as Galio, Brand, Veigar, etc. When the AP ratio of Exacerbation is included (between .2 and .26 - on average, around .23), their scaling is also comparable (.3+.25 becomes .53+.48, .5 becomes .73).
She has a lot of dashes (Q2, R2); keep that in mind when considering CC and durability - especially the Exacerbation active. Note that she has a persistent MS slow from Exacerbation's shield (WA), an AS/MS slow and silence from Preserve Specimen (E), and a brief AoE stun from Chronic Pain (Q2) - but minimal hard CC. For a number of characters with varying degrees of similarity, note characters like Galio, Diana, Evelynn, Fizz, Katarina, Lee Sin, Urgot, and Corki.
The 15% damage increase does not apply during most periods in lane - harrass with her Q1 won't be enough to dip targets below 80% health, and people take health potions. However, once she's forced heavy trading she earns a slight advantage (or, at least, works off her disadvantages). The second 15% damage increase is probably not going to kick into effect a lot of the time - imagine a situation where it would, and you'll probably recognise it's pretty much just when people are leaving.
Sara is designed to be played as an AP, AD, or hybrid assassin with some light bruiser aspects. She is extremely mobile as long as she's playing aggressively, and gains high offensive potential as a teamfight develops due to her passive abilities. However, her defensive abilities are limited to a shield based on her AP and a number of slows - it's hard to dash to safety.
She has a short stun akin to Riven's Ki Burst to catch people as she dives in with Chronic Pain, a persistent movement slow during her shield from Exacerbation, and a much more powerful but shorter movement and AS slow with her finishing move Preserve Specimen.
Her ult is meant to serve as a tool to follow up after initiation, dipping her first target into a realm of health that Sara likes to play with. With its percentage based damage, it serves to soften up a frontline (if she's playing keepaway for her AD carry); with its mid-range dash, it offers the ability to dive onto squishies (if she's taking an assassin role).
She has very minimal AoE besides proccing Exacerbation stacks (or Mortified Exacerbation); just Chronic Pain.