How do stats increase at level up?
Without equipment or other interference, the value of a hero's stat at level is
stat = floor( (.8 + level / 50) * level * ((a99 - a0) / 275.222) + a0 + .1 )
where a0 is the value of the stat at level 0, and a99 is at level 99.
Of course, stats can vary from this formula due to equipment, use of set hero stat and out of battle use of stat increase items. So when a hero levels, the difference between the calculated stat at the previous level and the new level is found and added to the actual stat value.
If the simulate old level-up bug bitset is on, then the stat modification of all equipment is added on as well - so if you level up 10 times with a +2 DEF helmet, you will gain 20 DEF, plus 2 DEF with the helmet equipped.
A stat growth graph is roughly parabolic. Here is a table of stat growth for a0 = 0, a99 = 1000:
lvl 0 = 0 lvl 1 = 3 lvl 2 = 6 lvl 3 = 9 lvl 4 = 13 lvl 5 = 16 lvl 6 = 20 lvl 7 = 24 lvl 8 = 28 lvl 9 = 32 lvl 10 = 36 lvl 11 = 41 lvl 12 = 45 lvl 13 = 50 lvl 14 = 55 lvl 15 = 60 lvl 16 = 65 lvl 17 = 71 lvl 18 = 76 lvl 19 = 82 lvl 20 = 87 lvl 21 = 93 lvl 22 = 99 lvl 23 = 105 lvl 24 = 112 lvl 25 = 118 lvl 26 = 125 lvl 27 = 132 lvl 28 = 138 lvl 29 = 146 lvl 30 = 153 lvl 31 = 160 lvl 32 = 168 lvl 33 = 175 lvl 34 = 183 lvl 35 = 191 lvl 36 = 199 lvl 37 = 207 lvl 38 = 215 lvl 39 = 224 lvl 40 = 233 lvl 41 = 241 lvl 42 = 250 lvl 43 = 259 lvl 44 = 269 lvl 45 = 278 lvl 46 = 288 lvl 47 = 297 lvl 48 = 307 lvl 49 = 317 lvl 50 = 327 lvl 51 = 337 lvl 52 = 348 lvl 53 = 358 lvl 54 = 369 lvl 55 = 380 lvl 56 = 391 lvl 57 = 402 lvl 58 = 413 lvl 59 = 425 lvl 60 = 436 lvl 61 = 448 lvl 62 = 460 lvl 63 = 472 lvl 64 = 484 lvl 65 = 496 lvl 66 = 508 lvl 67 = 521 lvl 68 = 534 lvl 69 = 547 lvl 70 = 560 lvl 71 = 573 lvl 72 = 586 lvl 73 = 600 lvl 74 = 613 lvl 75 = 627 lvl 76 = 641 lvl 77 = 655 lvl 78 = 669 lvl 79 = 683 lvl 80 = 698 lvl 81 = 712 lvl 82 = 727 lvl 83 = 742 lvl 84 = 757 lvl 85 = 772 lvl 86 = 788 lvl 87 = 803 lvl 88 = 819 lvl 89 = 834 lvl 90 = 850 lvl 91 = 866 lvl 92 = 883 lvl 93 = 899 lvl 94 = 915 lvl 95 = 932 lvl 96 = 949 lvl 97 = 966 lvl 98 = 983 lvl 99 = 1000