Disclaimer the first: this is a long, math-filled post. Ignore if these things are not interesting to you!
Disclaimer the second: math was my worst subject in school. Playing D&D has greatly improved my skills, but there's a lot of numbers here, so there could easily be a math error. Oops!
It's well known that I'm not terribly impressed with the AD&D Thief class; until levels beyond 7, it really seems to lag behind every other class, with terrible combat abilities, terrible Non-Weapon Proficiencies (ie, lowest acquisition rate at ¼ levels, only vaguely compensated for by their quicker level progression), and a low chance to use most Thieving Abilities.
Backstab can be so difficult to use if your DM is a stickler for the letter of the law that you might as well erase it from your character sheet in some games.
But I got to thinking, what would the most optimized version of the Thief look like? Here's my take...but a quick note. This isn't going to be useful in every game, I'm assuming an 18 Dexterity and certain minimum ability score requirements, as well as being allowed content from the Complete Bard's Handbook and from Warriors and Rogues of the Realms (oh and a small bit from the Complete Book of Elves as well).
Disclaimer the third: if you hate the idea of optimization, stop reading, please! This is for fun, since a few assumptions are being made, but I will state I don't think there's anything wrong with this character, since you are making sacrifices to be the best Thief you can be.
Obviously, your DM needs to allow those books, as well as be on board with this character concept in general; a Thief who is great at thieving, but bad at most everything else, who mostly levels up at the same speed as a Wizard and only has lesser spellcasting. They aren't great at combat, they don't wear armor, and they can't backstab.
So what's going on here?
The Complete Bard's Handbook has as section on Demi-Human Bards, and it covers Multiclassing with Bard and other classes, expanding on the rules in the PHB. Despite everywhere else disallowing multiclassing within the same group, this book allows for a few Thief/Bard options, though with certain limits (like having to take a particular Bard Kit).
Half-Elves, however, are allowed to be True (that is, normal) Bard-Thieves.*
*Theoretically, the True Bard, despite being the standard Bard is treated as a Kit in this book; for the purposes of this exercise, I'm assuming the DM won't rule that you aren't forced to take a Kit that does nothing!
The Complete Book of Elves (which includes Half-Elves) states under Kits that Multiclassed Elves can have a single Kit from any of their component classes. It specifically calls out “for example, a mage/thief can use a mage kit, a thief kit, or a mage/thief kit” on page 82.
With that said, we go looking for a good Thief Kit, and in Wizards and Rogues of the Realms, we find some very interesting choices for a lot of neat Thief builds, but for our purposes, we want to be a Rogue of the Realms- specifically a Rogue of Deepingdale.
The benefits of this Kit are immense; you gain a +25% bonus on three Thieving abilities, Pick Pockets (meh), Open Locks (great), and Find/Remove Traps (fantastic). Further, the benefits for not wearing armor are doubled, giving you +10% Pick Pockets, +20% Move Silently, +10% Hide in Shadows, and +20% to Climb Walls.
The downside, however, is a standing -1 on attack rolls and the loss of Backstab.
We're a Half-Elf, which is all upside; +10% Pick Pockets and +5% Hide in Shadows.
For this exercise, as mentioned, I'm going with 18 Dexterity. We also need 13 Intelligence and 15 Charisma.
So how does this work? Being a level 1 Thief gives us:
Pick Pockets 15%
Open Locks 10%
Find/Remove Traps 5%
Move Silently 10%
Hide in Shadows 5%
Detect Noise 15%
Climb Walls 60%
Read Languages 0%
You gain 60 points to place among these abilities, no more than 30% to a given one. On gaining a Thief level, you gain 30 more points, with a maximum of 15 points to a given ability.
Now, as a level 1 Bard, you have:
Climb Walls 50%
Detect Noise 20%
Pick Pockets 10%
Read Languages 5%
You gain 20 points to divide among these four (no limit) and 15 additional points on level up.
There are a few interpretations on how this works for this multiclass. I assume the following:
*You gain the most favorable base % of the two classes.
*You cannot combine points from both classes. Thus, only Bard discretionary points will ever be added to Climb Walls, Detect Noise, Pick Pockets, and Read Languages.
I assume that if you were to spend points from both classes on the same ability, only the best result would apply. There is a more favorable ruling, that you can freely mix these points, and a slightly worse ruling, that if you have Climb Walls from two sources, you would always use one or the other.
So our result, once we add up all the above bonuses (including no armor) is:
Pick Pockets 75%
Open Locks 60%
Find/Remove Traps 65%
Move Silently 40%
Hide in Shadows 30%
Detect Noise 45%
Climb Walls 85%
Read Languages 5%
To this, I'll first add the Bard 20 points as evenly as possible, 5 to each of the four thieving abilities. Read Languages isn't great at this point, but eventually it might come in handy. You could get lucky and find Comprehend Languages to add to your spell list, but you don't get many spell slots, so this can still be useful.
Then our 60 Thief points. Here I'm being more discerning; while being a great scout is useful, at first, I'm going to focus on being a door opening, trap finding Thief, though I am dropping points in Detect Noise, because knowing if there's an enemy on the other side of a locked door is really nice, so 20 points to each of these.
No, you know what, I'm changing my mind, 10 points to Open Locks, and 25 points to the other two.
Pick Pockets 70%
Open Locks 60%
Find/Remove Traps 65%
Move Silently 40%
Hide in Shadows 30%
Detect Noise 55%
Climb Walls 85%
Read Languages 10%
Now if you compared this to a level 1 Halfling Thief with the same Kit, and assumed that 19 Dexterity, they'd look like this:
Pick Pockets 70%
Open Locks 60%
Find/Remove Traps 45%
Move Silently 55%
Hide in Shadows 45%
Detect Noise 20%
Climb Walls 65%
Read Languages -5%
Adding the 60 points, the rules note we have to bring Read Languages to at least 1%, so that costs us 6 points. Of the remaining 54, that's 10 to Open Locks, 22 to Find/Remove Traps, and 22 to Detect Noise.
Pick Pockets 70%
Open Locks 70%
Find/Remove Traps 67%
Move Silently 55%
Hide in Shadows 45%
Detect Noise 42%
Climb Walls 65%
Read Languages 1%
Looks better, I know. And when the Halfling hits level 2 and gets their 30 extra points, they'll be even better. But things take a turn quickly.
At level 3 (2500 xp), our Halfling has a total of 120 bonus points. Our Bard-Thief has...125 (80 at level 1, 45 from level 2).
The progression up until level 7 goes like this:
120 3 vs. 125 2/2
150 4 vs. 170 3/3
180 5 vs. 215 4/4
210 6 vs. 260 5/5
240 7 vs. 305 6/6
And while our Halfling retains his x3 Backstab, our Half-Elf has the benefits of being a Bard at this point, which include: Singing proficiency, Musical Instrument proficiency, Reading/Writing proficiency, Local History proficiency (all free), 1st and 2nd level Wizard spells, situational abilities like inspiration, countersong, influencing NPC reactions, and a 30% chance to identify an unknown magic item in some way.
At 70,000 xp when the Halfling is level 8, they pull ahead a bit on levels, since it takes a whopping 140,000 xp for the Half-Elf...at which point the Halfling is level 9, but the Half-Elf is still ahead on bonus points (300 vs. 360) and now has the potential for 3rd level spell use.
So from here on out, the progression looks like this:
160,000 (H 10, H-E 8/8): 330 vs. 405
220,000 (H 11, H-E 9/9): 360 vs. 450
320,000 (H 11, H-E 10/10): 360 vs. 495
440,000 (H 12, H-E 11/11): 390 vs. 495
660,000 (H 13, H-E 11/11): 420 vs. 495
880,000 (H 14, H-E 12/12): 450 vs. 540
1,100,000 (H 15, H-E 15, H-E 12/12): 480 vs. 585
1,320,000 (H 16, H-E 13): 510 vs. 620
And so on. Now you might be asking if bonus points are relevant at this point, and likely not; even though you can continue to add points to abilities beyond 95% as per the Complete Thieves' Handbook, that's only sufficient to counter penalties; without magic items, you're generally capped at 95% success.
At this point, the saving grace of our “ultimate Thief” is actually their Bard spell progression, ability to use a wider variety of magic items, and slightly better ability to use scrolls. There is the question of followers the Halfling gets theirs first which is a big boon, and while the Half-Elf gets them with another 160,000 xp, there is the addition of 10d6 0-level soldiers, for what that's worth.